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There has never been a war like the one in the Pacific between the American Allies and the Empire of Japan. Unrivalled in its scope, it was a clash of cultures that turned tropical islands into killing grounds and laid waste cities with weapons of mass destruction. It turned World War II into a global war and ended with Japan's surrender. Here, an expert panel of ex-servicemen and military historians describe with clarity and dramatic detail each step of the conflict, providing a unique account of the myriad operations and underlying strategy of the bitter struggle to defeat Japan.

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Bernard C. Nalty


Thomas C. Hone


Daniel R Mortensen


Jacob Neufeld


George M. Watson Jr.


Edward J. Drea


Edward J. Marolda


Bernard C. Nalty


Wayne Thompson


William T. Y’Blood


William T. Y’Blood


William T. Y’Blood


Bernard C. Nalty


Bernard C. Nalty


William T. Y’Blood


Jeffrey G. Barlow


Jacob Neufeld


Bernard C. Nalty



Although the vast Pacific Ocean separated Japan from the United States, the interests of the two nations remained tightly intertwined for the better part of a century. In 1853, the visit of an American naval squadron provided the catalyst for the modernization of Japan. In 1945, after a bloody war, an instrument of surrender, signed on the deck of an American battleship, propelled Japan into a democratic era.

Commodore Matthew C. Perry dropped anchor in Tokyo Bay, then called Yedo Bay, in 1853 to deliver a demand from President Franklin Pierce for a treaty to ensure trade and to extend protection to shipwrecked seamen, whom the Japanese had killed or mistreated in order to preserve Japan from foreign corruption. The nation’s feudal rulers, impressed by the American flotilla, signed the agreement, but opposition to the pact rapidly coalesced. The opponents pinned their hopes on the emperor, who, they believed, would revert to a policy of isolation. Ironically, the proponents of change succeeded in using the office of emperor to legitimize the modernization of Japan. An oligarchy created a centralized government, nominally headed by an all-powerful emperor but actually dominated by this small group of advisers. Under the Emperor Meiji, the first ruler of modern Japan, who acceded to the throne in 1868, the nation took on the trappings of a constitutional monarchy, on the European model, but the oligarchs who wrote the constitution made sure that the Diet, or national legislature, had no real control over policy. Until the summer of 1945, when Emperor Hirohito shattered precedent to save his nation from destruction, Japan followed a course shaped by the cadre of military and civilian advisers who happened to be closest to the throne.

The new Japan promptly began expanding beyond the confines of the Home Islands of Honshu, Kyushu, Shikoku, and Hokkaido. The Japanese first claimed the Bonin Islands and Okinawa in the south and then cast covetous eyes upon the Kurile Islands and Sakhalin to the north, where Russia had a conflicting interest. The two nations compromised, with Japan taking over the Kuriles and Russia Sakhalin. Another object of Japanese ambitions, Korea, lay within China’s sphere of influence; but in 1894, Japan overthrew the Korean government, appointed a puppet regime, and defeated the Chinese forces trying to restore the old order.

Whereas Japan expanded step by step, the United States burst on the Far Eastern scene with dazzling suddenness. Victory over Spain in 1898 propelled the United States into world prominence as the master of an empire stretching from Puerto Rico to the Philippines. The island of Cuba where the United States had intervened to bring independence from Spain, became, in effect, an American protectorate. The peace treaty signed at Paris in 1898 contained a combination of Spanish indemnities and American payments that left the United States in control of Puerto Rico, conquered by American troops; Guam, where an expedition bound for the Philippines had put in and raised the flag; and the Philippine Islands, where some seven years of campaigning proved necessary to suppress an independence movement and impose control.

The destruction of Spain’s Pacific Empire – Germany purchased what did not go to the United States – coincided with the collapse of Chinese authority on the Asian mainland, a decline foreshadowed when Japan wrested Korea from the hands of the Chinese. Events in China spawned conditions that resulted in military cooperation between the two emerging Far Eastern powers, the United States and Japan. In 1900, as imperial power crumbled under foreign pressure, a group of militant Chinese, the Righteous Fist of Harmony, nicknamed the Boxers, attempted to expel the foreigners. The Boxers laid siege to the foreign legations at Peking (Beijing), and when the Chinese government, which secretly supported the uprising, made no move to protect the foreigners, a hastily organized international expedition, including Americans and Japanese, set out to the rescue. The relief column, however, soon found itself cut off from the other foreign troops at Tientsin (Tianjin) and a force of 18,000 men from a half-dozen nations had to be dispatched to break the sieges of Tientsin and then Peking. At Tientsin, Japanese troops broke through and led the other contingents into the city, but at Peking the Russians spearheaded the attack.

Japan soon was at odds with two of the nations that had helped it suppress the Boxers – Russia and the United States. Even as Japanese and American troops soldiered together in the international expedition, questions of race and competition for jobs were souring relations between the two powers. A treaty signed in 1894 resulted in a reciprocal grant of most favoured nation status. As a result, Japanese enjoyed the right to enter the United States, though not the privilege of becoming naturalized citizens. Moreover, a clause in the agreement enabled the United States to exclude labourers who might compete with Americans for jobs. Repeated American protests that most Japanese immigrants belonged in this prohibited category persuaded Japan to impose voluntary restrictions in 1900, but labourers continued to enter the continental United States by way of Canada, Mexico, and even Hawaii. Consequently, a Japanese and Korean Exclusion League came into being on the Pacific Coast, where prejudice against Asians was strong and competition for jobs fierce. In the fall of 1906, when the schools of San Francisco reopened after the disastrous April earthquake, all Asian pupils were segregated in a special Oriental school. The Japanese government protested that this action violated the treaty of 1894, and President Theodore Roosevelt fashioned a compromise whereby the San Francisco school board rescinded its order, the United States Congress enacted an immigration law empowering the federal government to bar any immigrants – clearly the Japanese – who might adversely affect “labor conditions”, and Japan entered into a “Gentlemen’s Agreement” to cooperate in preventing Japanese workers from emigrating to the United States.

Throughout much of the bitter agitation over racial discrimination against Japanese on the West Coast of the United States, Japan faced a more dangerous threat closer to home, as the military rivalry with Russia reached flashpoint. Although briefly allied against the Boxers, the two nations competed to fill the void caused by the decline of China. In 1895, five years before the relief of the besieged embassies at Peking, China had yielded to pressure and granted Russia the rights to Port Arthur (Lushun), near Darien (Luda) on the Liaotung (Liaodung) Peninsula. In 1902 Japan sought to neutralize the Russian move by entering into an alliance with Great Britain, which for some 20 years remained the keystone of Japanese foreign policy.

Japan soon concluded, however, that diplomacy alone could not provide security against Russia, and in 1904, without a formal declaration of war, the Japanese navy attacked the Russian squadron at Port Arthur, winning the first in a series of victories. The most spectacular success occurred in the Strait of Tsushima, where in May 1905 a Japanese fleet annihilated the outnumbered Russian Baltic Squadron, its equipment in disrepair after an exhausting voyage halfway around the world. Among those who fought that day under the victorious Adm. Heihachiro Togo was a young midshipman, Isoroku Takono, later adopted by the Yamamoto family, who lost two fingers of his left hand when an overheated gun exploded. Yamamoto would be a key figure in shaping and executing Japanese strategy in World War II.

Neither the overwhelming triumph of Togo, nor the victories at Port Arthur and Mukden, forced Russia to surrender. Lengthening casualty lists and mounting costs placed Japan in increasing peril as an inefficient but powerful enemy began bringing its resources to bear. When President Theodore Roosevelt offered to mediate, the war-weary combatants accepted his offer, Japan because the nation was being bled white, and Russia because of the danger of revolution at home. The American Chief Executive admired Japanese success against a potentially more powerful enemy – he had an instinctive sympathy for the underdog – and he may also have been embarrassed by California’s persistent discrimination against Japanese. Whatever his motivation, however, he believed that Japan received fair treatment in the peace settlement he helped fashion at Portsmouth, New Hampshire, in 1905. The Treaty of Portsmouth, which earned Roosevelt the Nobel Prize for peace, undoubtedly saved Japan from economic ruin and may also have staved off social disintegration in Russia, but it did not satisfy the government in Tokyo. Japan received hegemony over northeastern Asia and possession of Port Arthur and the northern half of Sakhalin Island, but did not get the large cash indemnity that it desperately wanted, and for this it blamed Roosevelt.

The president now realized that Japan had become the dominant power in the Far East and a threat to the newly acquired Philippines. Moreover, Japanese immigration to the United States remained a source of friction. As a result, Roosevelt sent the American battle fleet on a cruise around the world, from December 1907 to February 1909, as a show of strength, and followed up the “Gentlemen’s Agreement” to restrict immigration from Japan with a pact that in effect confirmed the dominant Japanese interests in northeast Asia, in return for recognition of American control over the Philippines.

Both the United States and Japan sided with the Allies in World War I. For Japan, the conflict presented an opportunity to absorb the German holdings in China and the islands in the central Pacific that Germany had bought from Spain. Whereas Japan acquired territory, the United States sought to restore the balance of power in Europe, described grandilo-quently as making the world safe for democracy, and had no specific territorial ambitions. The Treaty of Versailles that officially ended the war in 1919 awarded Japan trusteeship over Tinian and Saipan in the Marianas, as well as the Palaus, the Carolines, and the Marshalls; an arrangement confirmed by mandate of the postwar League of Nations. Having taken over the German holdings in China, the Japanese succeeded in asserting economic domination over Manchuria, all the while paying lip service to the Open Door – the policy of ensuring equal economic opportunity in China for all foreign nations, which had become a shibboleth of American foreign policy.

Japan thus emerged from World War I as an empire. The islands entrusted to it by the League of Nations lay athwart the lines of communication between Hawaii and the Philippines, which was a cause of concern to the United States. Great Britain, exhausted by the recent war, worried that the emergence of Japan as a major naval power would jeopardize its colonies of Hong Kong, Malaya and Burma. China, threatened more directly by Japan, feared for its existence. Against this backdrop, President Warren G. Harding of the United States convened the Washington Naval Conference in 1921. His Secretary of State, Charles Evans Hughes, hoped through negotiations among the interested powers to prevent a naval armaments race, keep Great Britain from siding with Japan in the event of a war in the western Pacific, and reduce the possibility of such a conflict. During the conference, the United States achieved everything it sought, in part because a group of cryptanalysts headed by Herbert O. Yardley had broken the Japanese diplomatic code, so that the American negotiators knew the instructions of the government in Tokyo almost as soon as their Japanese counterparts did. The Washington Naval Conference produced a treaty that established a tonnage ratio for capital ships – battleships, aircraft carriers, and cruisers – of 5:5:3 among the United States, Great Britain, and Japan and 1.7 for France and Italy. (Germany was forbidden any capital ships under the terms of the Versailles Treaty.) This apportionment of tonnage was designed to prevent any one nation from becoming the dominant naval power in the western Pacific. Since both the United States and the United Kingdom had naval commitments elsewhere, the two nations would have to act in concert if their fleets were to overcome the smaller Imperial Japanese Navy. As a further means of maintaining stability in the region, the treaty prevented the United States, Great Britain, and Japan from fortifying their outlying possessions. Excluded from this category was Singapore.

Pressure from the United States and Canada persuaded Great Britain to allow the alliance with Japan to lapse. To replace this bilateral pact, in 1921 the United States, Great Britain, France, and Japan fashioned a brand new agreement that guaranteed their territories, called for the peaceful resolution of differences among the signatories, and proposed joint action to meet aggression by any other power. These four powers took a further step to preserve peace in the western Pacific when they joined Italy, Portugal, Belgium, France, and the Netherlands in signing the Nine Power Treaty of 1922 which endorsed, but did not guarantee, the independence of China. In effect, this pact recognized the principle of the Open Door, which the United States and Great Britain had supported since the 1890s and Japan had more recently endorsed.

Although Japan accepted naval limitation and international cooperation in the early 1920s, the nation’s attitude hardened as the decade wore on. The ratio of 5:5:3 became a symbol of national humiliation, and was branded as “the navy’s failure” by a group of aggressive army officers intent upon seizing an economic bridgehead on the Asian mainland. Increasingly the army championed expansion at the expense of China, and the idea grew in popularity as the Great Depression spread world-wide and fixed its grip upon Japan. As a source of raw materials and a market for manufactured goods, China seemed to hold the answer to Japan’s economic woes.

Those who believed that conquest could restore the economy soon found themselves in a race against time. On the mainland, Chiang Kai-shek (Jiang Jieshi) seemed to be making progress toward unifying China; merging the feudal states, each with its own war lord, into a modern nation. This process, if brought to completion, boded ill for the Japanese and other foreigners who had staked out claims. In September 1931, Japan’s militarists struck. A bomb, allegedly planted by Chinese terrorists, exploded on the right of way of the Japanese-owned Manchurian railway, giving the Japanese army an excuse to invade Chinese territory to protect the property of Japanese citizens. Unable to oppose the invasion, China appealed to the League of Nations, which investigated the Chinese complaint, concluded that Japan had no justification for its action, but did nothing to redress the wrong. Although the League contented itself with the mildest of reprimands, Japan angrily withdrew from the organization. The United States lodged a protest through diplomatic channels, but the administration of President Herbert Hoover placed an infinitely higher priority on economic recovery than on preserving the territorial integrity of China.

The conquest of Manchuria, which became the Japanese territory of Manchukuo, reflected the ascendancy of the most militant elements of the army in Japanese politics. The radical army officers intended to control the government and manipulate Emperor Hirohito, who had ascended the throne in 1926, as they led Japan into a golden age of military and economic dominance in Asia. Some of the military men, like Hideki Tojo, who would serve as prime minister during much of World War II, were prepared to work within the existing political and economic structure, but others had less patience. The true firebrands proposed nothing less than the overthrow of the industrialists and politicians who, they believed, served the emperor badly and prevented Japan from achieving true greatness.

In 1936, five years after the conquest of Manchuria, a group of Japanese army officers mutinied, killed two of Hirohito’s principal advisers, and wounded a third, Adm. Kantaro Suzuki, who lived to serve as prime minister in August 1945 as World War II drew to an end. The high command did not rally behind the plotters, who had gone too far for most of the senior officers. The task of settling the issue without compromising the honour of the officer corps fell to Maj. Gen. Tomoyuki Yamashita, who would achieve prominence in World War II as the conqueror of Malaya. Yamashita tended to be more radical than Tojo, and for that reason he proved an effective intermediary in persuading the leaders of the mutiny to commit ritual suicide, thus avoiding a trial that might have demoralized the army.

The Imperial Japanese Navy showed less enthusiasm than the army for waging a war of aggression, possibly because the soldiers thought mainly in terms of a war on the mainland of Asia against Chinese forces that were almost all ill-trained and poorly armed, while the navy planned for a war against the American Navy backed by the industrial might of the United States. Indeed, a strong faction within the naval establishment continued to believe in naval limitation, although with a tonnage ratio more favorable to Japan. The so-called “Treaty Faction” included Yamamoto, who by 1930 was clearly on the fast track to flag rank.

During the 1920s, Yamamoto had served two tours in the United States, one as an assistant naval attaché at Washington and the other as a language student at Harvard University. At the capital, he became interested in air power. During his assignment to the attaché’s office, the United States Navy experimented with its first aircraft carrier, a converted collier rechristened USS Langley, and Brig. Gen. William L. “Billy” Mitchell of the Army air service sank a number of ships, including obsolete battleships, anchored as targets for land-based bombers. Inspired by these developments, Yamamoto came to believe that airplanes and aircraft carriers were the weapons of the future.

While studying at Harvard, Yamamoto attended few formal classes but roamed North America from the automobile factories of Detroit to the oil fields of Texas and Mexico, an experience that impressed him with the industrial resources and mineral wealth at the disposal of the United States. He became convinced that a war with the United States would prove disastrous for Japan. Japanese diplomacy, he maintained, should seek a balance of power in the Pacific that would deter either nation from attacking the other. For Yamamoto, agreed tonnage ratios afforded the likeliest means of maintaining that equilibrium.

The views of the Treaty Faction did not prevail. The government of Japan dared not extend the agreement on the 5:5:3 ratio, which the populace had come to see as a proof of submission to the United States and Great Britain. A change in the ratio to increase Japanese tonnage might have saved the principle of naval limitation, but the British and American governments would make only minor concessions. In 1930, Yamamoto served as technical adviser to a Japanese negotiating team that obtained agreement to a five-year extension that permitted Japan to build as many submarines as the Americans or British and established a 10:10:7 ratio for cruisers. The hated proportion of 5:5:3 continued, however, to prevail for battleships and aircraft carriers. As the pact signed in 1930 drew to an end, Yamamoto took part in another attempt to modify the old ratio; the Treaty Faction empowered him to propose the abolition of the aircraft carrier in the hope of jump-starting the talks, but to no avail. Unable to obtain a satisfactory ratio, Japan broke off the talks and in 1937 approved the construction of the world’s largest battleships, the 72,000-ton Yamato and Musashi.

Having cast aside naval limitation and launched a program of warship construction, Japan began searching for allies. Since the Soviet Union posed a threat to Manchukuo, in 1936 Japan aligned itself with Nazi Germany and Fascist Italy in the Anti-Comintern Pact, an agreement that imposed no military obligations but merely called for cooperation in preventing the export of Soviet Communism. While Japan’s diplomats sought to protect Manchukuo by adhering to the Anti-Comintern pact, the army used the territory as a springboard for an assault on China. Japanese haughtiness and Chinese pride rendered such a conflict all but inevitable; firing erupted at the Marco Polo bridge near Peking in July 1937, and the war began.

The administration of President Franklin D. Roosevelt tried to maintain correct, though not cordial, relations with Japan, in the hope that the mounting costs of a war in the vastness of China would discredit the militarists and bring to power a government willing to write off the venture as a bad gamble. Unfortunately, the United States had little influence on Japanese behavior, as demonstrated in December 1937, when naval aviators sank the clearly marked American gunboat, USS Panay, as it escorted a convoy of Standard Oil tankers, and then machine-gunned the crew members who sought concealment among the reeds on the banks of the Yangtze (Chang Jiang) River. Two sailors and a civilian passenger died as a result of the attack, and 11 members of the crew were wounded. Japanese authorities sent ships to pick up the survivors and promptly apologized for what they described as a tragic accident. Yamamoto himself, who was now a vice minister in the navy ministry, promised to take “redoubled precautions” to prevent a similar incident in the future. The Japanese response served to defuse a potentially explosive situation, but less dramatic confrontations continued to occur as Americans tried to conduct business as usual in the midst of an increasingly violent war.

The mounting tensions in the Far East coincided with a growing danger in Europe, where Adolf Hitler consolidated his power over Germany in 1933 and 1934, began a systematic persecution of the nation’s Jews, and rearmed in defiance of the Treaty of Versailles. He intervened in the Spanish Civil War and by the summer of 1939 absorbed Austria and Czechoslovakia. A non-aggression treaty with the Soviet Union gave Hitler the opportunity to attack Poland, but to his surprise France and Great Britain came to the aid of the Poles, and World War II began in September 1939. By the end of June of the following year, German forces had overrun western Europe from Norway to the Pyrenees, driving the British from the continent. During 1941, Hitler overwhelmed Yugoslavia and crushed the resistance of the Greeks, who had checked an invasion by his Italian ally. He also launched an attack across the North African desert toward the Suez Canal and in June attacked the Soviet Union. The American government concluded that Hitler’s Germany was the most dangerous potential enemy, and President Franklin Roosevelt sought to marshal the nation’s industries to aid the foes of Nazism.

Although Hitler seemed the greater threat to world order, Japan could not be ignored. The Roosevelt administration continued to use diplomatic and economic pressure in an attempt to restrain the Japanese. The United States government allowed the commercial treaty with Japan to expire, extended credit to China to help finance resistance to the Japanese invasion, and imposed an embargo on a lengthening list of war materials destined for Japan, including aluminum for aircraft production and aviation gasoline. Protests and material shortages could not, however, have a decisive effect on Japan’s determined militarists who had already demonstrated their willingness to kill their opponents.

In April 1940, before Hitler’s army had advanced to the Channel coast, Roosevelt decided to reinforce the diplomatic and economic measures with a modest show of force. He directed that the United States Fleet, after completing its annual exercises, remain in Hawaiian waters, shifting its home port from San Pedro, California, to Pearl Harbor. “Why are we here?” asked Adm. James O. Richardson, the fleet’s commander, who objected to massing his ships at the end of a vulnerable supply line stretching halfway across the Pacific. “You are there,” replied Adm. Harold R. Stark, the Chief of Naval Operations, “because of the deterrent effect it is thought your presence may have on the Japs going into the East Indies.”

As Stark’s reply indicated, the German conquest of France and the Netherlands, along with the British withdrawal from the continent, afforded Japan an opportunity to seize the colonies of the Netherlands East Indies, French Indochina, and British Malaya and Burma.

Indeed, following the collapse of France in June 1940, Japan extorted the first in a series of concessions in Indochina that soon included the right to build airfields, access to the rice harvest, and use of the airport at Saigon (Ho Chi Minh City).

The American economic restrictions were having an effect, though not the desired one of inspiring a sober reconsideration of Japan’s expansionist policy. Indeed, Japanese tended to see American concern for the Open Door and support of China, which the trade measures dramatized, as especially offensive examples of meddling in Japan’s natural sphere of interest. In the summer of 1940, when Hitler’s triumph in western Europe persuaded the United States to embark on a massive program of naval construction, the Japanese assumed that the resulting “Two-Ocean Navy” would be used against them. A naval arms race with the United States loomed on the horizon.

As Japanese-American relations thus deteriorated, the Anti-Comintern Pact no longer afforded protection against Soviet designs on Manchukuo, for in August 1939, Germany and the Soviet Union had signed their non-aggression pact. In these circumstances, Japan sought to neutralize the threat to Manchukuo, for Japanese and Soviet troops had clashed in 1938 and 1939 on the border between Manchukuo and the Soviet Republic of Mongolia. The answer seemed to lie in a military alliance with Hitler, while reaching an accommodation with Joseph Stalin. In September 1940, Japan, Germany, and Italy acceded to the Tripartite Pact, by which they agreed to go to the aid of any partner attacked by a power not already involved in the European conflict or the war in China. Japan obtained further assurance of a free hand in Asia by negotiating a non-aggression treaty with Soviet Union, signed in April 1941.

Although the Japanese army tended to applaud the alliance with Hitler and Benito Mussolini, the Italian leader, elements within the navy opposed the idea. The Navy Minister, Adm. Mitsumasa Yonai, warned that his service, still too weak to defeat the fleets of Great Britain and the United States, could expect no help from Germany or Italy, whose navies were fully committed in the Mediterranean and Atlantic. The alliance with Germany and Italy, he pointed out, would antagonize the United States and Great Britain without reinforcing Japanese naval strength in the Pacific.

The attitude of the naval leadership began to change, however, as symbolized by the conversion of Adm. Yamamoto from an opponent of a war with the United States to an architect of Japanese strategy. As Japan drew closer to Germany, Yamamoto warned that: “A war between Japan and the United States would be a major calamity for the world, and for Japan it would mean, after several years of war already [in China], acquiring yet another powerful enemy – an extremely perilous matter for the nation... It is necessary, therefore, that both Japan and America should seek every means to avoid a direct clash, and Japan should under no circumstances make an alliance with Germany.” But alliance was made, and Japan drifted closer to war with the United States. In these circumstances, Yamamoto became convinced that every Japanese, himself included, had to fall in line behind the emperor, whatever the risk to the nation. In April 1941, he wrote to a friend, “if by any chance there should be a war between Japan and America within the year, I am ready to carry out my duties in a way that’ll have you all saying ‘Good old Iso [roku]’.”

Before he wrote this letter, perhaps as early as January, Yamamoto concluded that he could best carry out his duties to emperor and country by attacking the American Pacific Fleet at Pearl Harbor, gaining time for Japan to seize the Philippines, Malaya, Burma, and the Netherlands East Indies and thus acquire an empire rich in oil, ore, rice, and other resources. Other officers, especially in the army, believed that Japan’s warrior tradition would prevail in such a conflict; the Americans, they said, lacked the discipline and courage to recover from the initial Japanese victories. The United States would not pay the price in blood and treasure to roll back the tide; the war would end in a negotiated settlement that accepted Japanese domination of the western Pacific.

Such were the predictions of the optimists, but Yamamoto entertained no such illusions. He had lived in the United States and knew firsthand of its overwhelming industrial power and vast natural resources. If war should come, he wrote, “I shall run wild for the first six months or a year, but I have no confidence for the second and third years.”

Instead of going to war in the expectation that the United States would lose heart, Yamamoto urged that Japan prepare for a long and bloody war and resolve to do whatever was necessary to win. Taking “Guam and the Philippines, nor even Hawaii and San Francisco,” would not be enough; Japan he said, “would have to march into Washington and sign the treaty in the White House.”

The road to war began running sharply downhill in July 1941, when Japan, after nibbling away at French Indochina, decided to bolt down what remained. President Roosevelt reacted by imposing an embargo on the shipment of American oil to Japan and then by freezing Japanese assets in the United States. The United Kingdom and the Netherlands government in exile also impounded Japanese funds, so that Japan could no longer buy oil from the usual Dutch or American suppliers. Imports of crude oil and petroleum products, which totaled almost 40 million barrels during the year ending in March 1941, slowed overnight to a comparative trickle from Latin America and the Near East. In the fall of 1941, Japan’s oil reserves amounted to some 50 million barrels, but the fleet burned 2,900 barrels during each hour’s steaming. The temptation to invest the reserve in a war of conquest that would gain the oil of the Netherlands East Indies proved irresistible. Economic pressure designed to discourage aggression actually accelerated the movement toward war.

Despite heightening tensions, the Japanese prime minister, Prince Fumimaro Konoye, still hoped to avoid hostilities and tried through diplomacy to persuade the United States to accept Japanese hegemony in the Far East. The army grew impatient, however, and decided that Konoye lacked the resolve needed to prosecute a war with the United States. After consultations with his principal advisers, including Minister of War Hideki Tojo, Konoye stepped aside, and on October 17, Tojo replaced him. At Washington, the Japanese ambassador, Adm. Kichisaburo Nomura, joined in November 1941 by a special emissary, Saburo Kurusu, persisted in trying to talk the United States into giving Japan a free hand, even though the two diplomats could offer no concessions. An agreement, moreover, had to be reached by a specific date, initially November 25 but postponed to the 29th, though the Japanese negotiators did not realize that the alternative would be war.

While Yamamoto, his navy colleagues, and the army leadership made their plans, and the diplomats pursued their talks, American intelligence was intercepting, decoding, and translating Japanese diplomatic traffic. During the summer of 1940, the Army’s Signal Intelligence Service, under Lt. Col. William F. Friedman, had succeeded in breaking the Japanese Purple code, a demanding task that resulted in Friedman’s nervous collapse and temporary hospitalization. As Purple decoding machines became available, one went to the Philippines, one to Hawaii, and two to London, while four remained at Washington. Friedman’s machines enabled the United States to eavesdrop on the Japanese Foreign Office, so that American officials sometimes read Nomura’s instructions from Tokyo before he did. No wonder the product of the cryptanalysts was called Magic. Other diplomatic codes less difficult than Purple also proved vulnerable to this kind of cryptanalytical assault.

Unfortunately, Japanese naval codes remained unbroken for the present. Unable to decipher Yamamoto’s messages to his fleet, the Americans had to try to draw conclusions from the volume and source of the traffic and the call signs and transmitting characteristics of the radio operators. This technique was vulnerable to rudimentary countermeasures, such as changing call signs or imposing radio silence, which masked the sailing of the Pearl Harbor task force and its approach to the objective.

As the Japanese steamed toward Hawaii, peacetime routine prevailed on the ships of the Pacific Fleet. The usual Saturday morning inspection took place on December 6. Rear Adm. H. Fairfax Leary checked the shaves, haircuts, and uniforms of the sailors of the light cruiser Phoenix and then made a white glove inspection of the ship. On board the battleship California, the crew prepared for a more extensive inspection scheduled for the coming week by removing the covers to a half-a-dozen hatches leading to the double bottom. When the Japanese attacked on December 7, Phoenix came through undamaged except for a single bullet hole in the superstructure; California, however, flooded when hit by deep-running torpedoes, in part because of the open hatches, and settled into the mud of Pearl Harbor.


Japan’s attack on Pearl Harbor should not have surprised senior officers of the United States Navy. Not since England and France had fought for control of the seas in their colonial wars of the eighteenth century had two nations planned so assiduously for a clash at sea as Japan and the United States in the years before World War II. Once the United States became a colonial power in Asia, after the Spanish-American War, the US Navy had to plan to hold the Philippines in the face of threats from foreign powers. After World War I, the only nation with a navy able to challenge the US Navy in the Pacific and with any reason to do so was Japan. Japan controlled resources in Korea and wanted control over Manchuria and, eventually, China. The United States consistently opposed Japanese policy. The real question was not whether the navies of the two nations would fight, but when.

Between the end of the World War I and the termination, by the government of the United States in July 1939, of the commercial treaty linking the two nations, both navies planned carefully for a trans-Pacific war. Their strategic situations were quite different. Japan was Asia’s Britain – an island kingdom vulnerable to blockade and starvation. To make matters potentially even worse, Japan’s merchant fleet was not as large as Britain’s, and her industrial output was not quite the equal of Italy, let alone the United Kingdom. On the positive side, Japan’s likely enemy, the United States, was very far away. It is just over 4,500 nautical miles (8,100km) from San Francisco to Yokohama. The distance from San Diego, a major American naval base on the Pacific coast, to Manila is 6,500 nautical miles (11,700km). From Yokohama to Manila, on the other hand, is just about 1,750 nautical miles (3,150km). Pearl Harbor is nearly 4,700 nautical miles (8,460km) from the Panama Canal, 2,200 (3,960km) from San Diego, 3,300 (5,940km) from Guam, in the Marianas, and 1,970 (3,546km) from Wotje, the Japanese anchorage in the Marshall Islands. These great Pacific distances dominated both Japanese and American naval planning between the first and second World Wars.

Given the industrial power of the United States, Japan’s only hope was to wage a war of attrition, wearing down American strength until the government of the United States accepted a compromise peace. A more aggressive strategy was blocked by Japan’s signing of the five-power naval limitation agreement when the Washington Conference ended in 1922. That agreement set ratios and limits to the warship tonnages allowed the signatories (Great Britain, the United States, Japan, France, and Italy). The ratio of battleship tonnage for Great Britain, the United States, and Japan was 5:5:3, which meant, in practice, that the United States Fleet would have 15 battleships to the Imperial Japanese Navy’s ten. Japan did somewhat better on cruiser tonnage, when the ratio among the three major navies was adjusted to 10:10:7. For aircraft carriers, then largely experimental, the ratio was the same as for battleships, with a cap of 135,000 tons for the United States and Great Britain and 81,000 tons for Japan. These limits forced the Imperial Navy to develop a defensive strategy for war with the United States.

The Versailles conference, which ended World War I, gave Japan jurisdiction over several former German possessions in the Pacific, including the Marianas (except Guam, which belonged to the United States), the Carolines, and the Marshalls. Soon afterward, the newly formed League of Nations affirmed the Japanese mandate over these islands, which stood athwart the direct route to the Philippines from Hawaii. Though Japan controlled them, successive Japanese governments pledged not to fortify them in exchange for a commitment by the United States not to modernize its defences in the Philippines. Possession of these islands was a two-edged sword. On the one hand, Japanese forces could use them to block any thrust by the United States Navy to the Philippines. On the other, the United States could seize bases there in the event of war and then use those bases as way-stations on the road back to the Philippines.

Both Japanese and American naval planners believed that the Philippines could not be held in the face of a concerted Japanese attack. The best the limited number of United States troops might do was to hold the island forts in Manila Bay, denying that anchorage to the Imperial Fleet. It was not clear, however, how long they might hold out, and around that question spun the major issues of United States Navy war planning. Should the United States try to reinforce the Philippines quickly, aiming for a quick, decisive confrontation with the Japanese fleet? Or, should the US Navy move cautiously, conquering anchorages and bases in the Marshalls and Carolines, gradually building up its strength until it could fight the Imperial Fleet in a showdown in Philippine waters? For the Japanese Navy, the new island possessions were welcome as bases (Truk, in the Carolines, for example, is a magnificent anchorage) but also a problem: they had to be supplied. In wartime, they might be isolated and cut off, leaving smaller contingents of the Imperial Fleet to face larger contingents of an attacking American force.

Distances were, as usual, everything. From Yokohama to Truk is over 1,800 nautical miles (3,240km); Yokohama to Saipan, in the Marianas, is almost 1,300 nautical miles (2,340km). Yokohama to Wotje, in the Marshalls, is more than 2,300 nautical miles (4,140km), which meant that an American task force could reach the Marshalls from Hawaii faster than reinforcements from Japan. Japanese Navy planners faced a dilemma. If they dispersed their smaller fleet, it might be defeated piecemeal by a concentrated and larger American force. If they held their forces back, however, waiting to strike the decisive blow, they might not be able to reach the scene of action in time to prevent the United States Navy (and its Marine Corps) from conquering a base that, properly reinforced, could serve as the next stepping stone on the way to the Philippines and the Japanese Home Islands. United States Navy planners had their own problems. The naval agreements which kept the Imperial Fleet smaller than the United States Navy also limited the immediate wartime potential of the latter. American interwar fleet exercises always showed that the United States Navy needed more ships and aircraft if it were to overcome Japanese forces at great distances from secure American bases. In the 1920s and 30s, Pearl Harbor lacked machine shops, a trained labor force, and adequate supplies of fuel to support a fleet thrust west-ward. Ships damaged by torpedoes and bombs would have to return to San Diego for repair, and replacement aircraft would have to be shuttled out in cargo ships to the advancing force across thousands of miles of open water, under constant threat of Japanese submarine or surface raider attack.

Planners in the two navies wrestled with these problems for years. Their conclusions shaped the characteristics of the ships built in peacetime for the coming trans-Pacific war. The United States Navy’s “treaty” cruisers, for example, built to the limits (10,000 tons and 8-inch guns) set by the naval arms control agreement signed in 1922, had great endurance and guns of great range, but they were lightly armored until the New Orleans class of 1930. Their Japanese counterparts, such as the Furutakas and Aobas, were built for speed, so they could decline battle if outnumbered. However, the latter were eventually equipped with very large (24-inch diameter), very long range (20 nautical miles – 36km), Type 93 torpedoes, each of which was – for a destroyer or light cruiser – a ship-killer. The reason was simple: the United States Navy had the advantage in the daytime, and the Japanese at night. On clear, central Pacific days, the US Navy’s advantage in battleship and cruiser broadside weight would eventually tell. To counter it, Japanese planners designed fast surface ships and an anti-ship torpedo which could wreak havoc by night upon American forces steaming in defensive formations.

Both navies were spurred by the need to overcome great distances and the limits on numbers and sizes of ships, which put a premium on packing as much fighting power into a ship as possible. These pressures also affected aircraft carrier design. At the Washington conference of 1921-2, for example, the US Navy and the Imperial Navy were allowed to convert two battlecruisers or battleships then building to large aircraft carriers. These ships eventually emerged as carriers Lexington, Saratoga, Kaga, and Akagi. The former US Navy battlecruisers were especially useful because of their great steaming ranges 10,500 nautical miles (18,900km) at 15 knots, and the ability to survive damage (Lexington was torpedoed twice at Coral Sea in May 1942 and still steamed at 25 knots). Their Japanese counterparts had less range – 8,000 nautical miles (14,400km) at 14 knots – but were otherwise similar. All four ships, though not designed as carriers from the keel up and therefore less than ideal, had the size to accept the increasingly large and heavy planes which their respective industries supplied as the interwar years passed.

Carriers commissioned after the conversions tended to be much smaller. The American Ranger, laid down in 1931, was only 14,500 tons standard displacement (the treaty measure). Ryujo, Ranger’s Japanese contemporary, was even lighter: 8,000 tons standard displacement. Both navies wanted to maximize the number of carriers that could be built under treaty limits. Exercises had shown air-minded officers in both fleets that, in battle, the side with the most (and most effective) planes in the air usually won. Accordingly, aviators pressed for more carriers; since more decks meant more planes in the air more quickly. However, the size of carriers depended upon the size and weight of the aircraft they carried, and planes kept getting larger and heavier in the years just before 1941. The Boeing F3B-1 biplane fighter of 1928, for example, weighed 2,950 pounds (1,180kg). The Grumman F4F-4 Wildcat of 1941 tipped the scales at 7,975 pounds (3,190kg). The Nakajima A2N1 of 1931 weighed 3,416 pounds (1,366kg). The Mitsubishi A6M2 Zero of 1939 was up to 5,139 pounds (2,055kg). Like the fighters, dive bombers and torpedo planes grew larger and heavier. Carriers had to increase in tonnage to operate larger, faster, more potent planes. Ranger was followed by Enterprise (laid down in 1934), of 19,800 tons; Ryujo was succeeded by Soryu (also laid down in 1934), of 15,900 tons.

As carriers and their planes grew larger and more powerful, carrier tactics also improved. In Fleet Problem IX, staged in 1929 near the Panama Canal, Saratoga proved that a large carrier could strike effectively and independently deep into enemy areas. From that time until 1941 – and even through 1942 – both navies worked to invent the best ways of deploying, maneuvering, and defending their aircraft carriers. In the 1930s carriers had to rely almost exclusively on their own antiaircraft guns and those of their escorts to shield them from an enemy’s air attack. This was not a really effective defence. As a result, in exercises, the carrier which found its opponent first usually delivered the knockout blow. Carriers were fragile. Their power was in their air groups, usually two or three squadrons of attack aircraft (dive bombers and torpedo planes) and one squadron of fighters. The ships themselves were floating magazines, filled with munitions and highly volatile aviation gas for their planes.

By 1939, carrier aircraft could conduct highly coordinated strikes on enemy carriers and other ships. Attacks could be mounted quickly, and planes often practiced pre-dawn take-offs, using short-range radio once airborne to tighten their formations. After 1937, Japanese carrier aircraft battled Chinese air units, gaining wartime experience in aircraft tactics and in the launching and recovery of fully armed and (sometimes) damaged planes. By the end of the 1930s, the striking power of carriers was acknowledged in both navies. Adm. Isoroku Yamamoto, later commander-in-chief of Japan’s Combined Fleet, made his reputation as an innovative leader in naval aviation. He firmly believed that long range and offensive power were the keys to victory in carrier-on-carrier battles, and he forced Japan’s aircraft manufacturers to build planes that flew farther and carried at least equivalent ordnance loads when compared with their US Navy counterparts. He helped mold a force well suited to taking the initiative at the beginning of a conflict.

Supporting the carriers were other forms of naval aviation. The US Navy had agreed with the US Army in 1931 not to develop land-based bombers, but the Navy had built up a force of long-range seaplanes, including, by 1941, impressive numbers of the PBY Catalina, which could (and early in World War II actually did) stage high-altitude formation bombing raids like those conducted by B-17s. The Imperial Japanese Navy had its own similar seaplanes, as well as an impressive land-based bomber force. As with its cruiser and destroyer designs, its land-based bombers reflected Japanese battle tactics. Assuming that the US Navy would launch an offensive to rescue the defenders of the Philippines, the Japanese navy planned to whittle American numbers down through submarine attacks, night raids conducted by land-based bombers, and torpedo assaults launched during the dark by groups of cruisers and destroyers. After wearing the United States Fleet down – equalizing the strength in battleships and carriers – the imperial fleet would wade in for the final, conclusive battle. A decisive Japanese victory there would leave the United States with little choice (so the argument ran) but to accept a compromise peace.

Unfolding events disrupted this planning. First, the Pacific Ocean no longer monopolized the interests of American strategists, who turned during the 1930 to the Atlantic as well. A rearmed Germany, allied with Italy, confronted France and Britain in Europe. The threat of war there forced the US Navy in 1939 to abandon its traditional war plan (called Orange) against Japan in favor of a series of plans (dubbed Rainbow) covering a number of possibilities and belligerents. In the crucial year of 1940, moreover, Japan fortified the Marshall Islands. The American President, Franklin D. Roosevelt, reacted by ordering the United States Fleet to remain in Hawaii after completing its annual war problem. In May and June of that year, German armies rolled over British and French forces, compelling both Washington and Tokyo to reassess their long-range plans. In response, the United States Congress authorized construction of what was called a “two-ocean” navy: 9 new battleships, 6 battle cruisers, 8 heavy cruisers, 34 light cruisers, 11 large aircraft carriers, 5 small carriers, 179 destroyers, 67 submarines, and a large force of support and amphibious ships. Justified at home by the potential threat from Germany, the proposed fleet was seen as something very different in Japan. There, the military leadership perceived the huge building program as part of a larger plan to give the US Navy clear superiority in the waters near Japan. In response, Japan joined the Axis (September 1940), taking advantage of Germany’s defeat of France to gain bases in French Indochina.

By the end of 1940, Adm. Yamamoto, now head of Japan’s Combined Fleet, thought seriously of abandoning the Japanese navy’s accepted strategy of a fighting, drawn-out defense against an American offensive. His thinking reflected a change in Japan’s strategic goals. The United States was just one of several potential enemies. The German government had informed Japanese leaders that Britain’s military officials believed that their forces were too slim to hold Malaya if Japan attacked. Assuming that were true, Malaya and the Netherlands East Indies provided opportunities for easy conquests that would give Japan a secure supply of oil and other needed resources, offsetting the constraining effect of the ever tightening American economic boycott. With the Royal Navy hard-pressed in the Atlantic and the Mediterranean, Singapore posed a diminished threat. The danger that the Imperial Fleet would have to fight a multi-front campaign while still trying to consolidate its conquest of the Philippines seemed much reduced. The position of the United States was also changing. In November 1940, President Roosevelt had accepted “Plan D”, based on the fourth paragraph, lettered D, of a proposal by Chief of Naval Operations Adm. Harold R. Stark that America’s primary goal, in the event of a world war, should be the defeat of Germany. The American decision to focus first on Europe gave Japan a great opportunity, and Adm. Yamamoto capitalized on it. In January 1941, the Combined Fleet formally requested the Imperial Navy Ministry to supply the Fleet’s carrier torpedo planes with weapons that could be launched in the 40-to-45-foot (12-13m) depths of Pearl Harbor. This proved quite a challenge. Aircraft torpedoes usually sank below that depth before stabilizing on shallow runs to their targets. However, Royal Navy torpedo planes had used shallow-running torpedoes in their attack on the Italian battle fleet in Taranto in November 1940, with spectacular results, so Yamamoto thought the gamble well worth the risk.

Also that January, the admiral revealed to a colleague his plans to change Japanese battle plans completely – to forego attrition in the mandated islands as the prelude to a decisive battle, and move offensively against the United States Fleet in Hawaii while attacking Singapore and the Philippines. By February, Yamamoto’s staff had formulated a draft plan for the Pearl Harbor operation, and the admiral submitted it to Cdr. Minoru Genda, a young aviator with a brilliant grasp of carrier aircraft tactics. Genda became convinced the attack would work, provided sufficient aircraft were massed for the operation. He also persuaded Yamamoto that the primary target of the attack should be the US Navy’s aircraft carriers, and he argued unsuccessfully for an amphibious assault on Oahu. Genda’s goal was a “knock-out blow”. His chief, however, remained content with a “knock-down blow”.

That same month, the United States Navy formally acknowledged the shift in American strategy toward Europe by dividing the United States Fleet into Atlantic and Pacific commands. In March, the military chiefs of the United States and Great Britain formally and secretly agreed to take a defensive but coordinated stand against further Japanese moves in the Pacific. They agreed on litde else. There would be no unified Pacific command, and no joint strategy. However, the Congress, at Roosevelt’s urging, approved the Lend-Lease Act in March, allowing Great Britain to tap US industry without paying for its purchases immediately in cash.

In April, Adm. Yamamoto formed the First Air Fleet, the striking force of Japan’s six largest aircraft carriers that would attack Pearl Harbor that December. In support of its move south and east, Japan signed a neutrality pact with the Soviet Union in April, protecting Manchukuo and Korea from attack and further directing the coming theater of war to the South Pacific. At the end of the month, Yamamoto submitted his plan for multiple offensives (including the Pearl Harbor attack) to the Imperial Fleet’s General Staff. His plans gained support from an American decision to shift three battleships, an aircraft carrier, and supporting cruisers and destroyers to the newly formed Atlantic Fleet. Adm. Husband Kimmel was no longer in charge of the United States Fleet. The ships that remained under his command now constituted the Pacific Fleet, but Kimmel and his staff did not think the force in Hawaii could deter Japan from a move against the Philippines or Southeast Asia.

President Roosevelt froze Japan’s assets in the United States in July, announced that the United States would send a military mission to China, and cut the flow of oil to Japan to a trickle. His goal was to influence Japan without provoking her. He failed. He had no influence whatsoever with the men who now mattered: the staff officers in Tokyo analyzing Yamamoto’s plans. In September, a series of war games, in which the senior Japanese naval officers took part, tested Yamamoto’s proposals. Based on the results of the games, the General Staff recommended against the Pearl Harbor attack. The Imperial Navy had only 11 carriers. In sending the six best against Pearl Harbor, Yamamoto was risking everything on one throw of the dice. Moreover, in striking at Pearl Harbor, Yamamoto was placing heavy burdens on the air and naval units left to cover the invasions of the Philippines, Malaya, and the Netherlands East Indies. Equal losses at Pearl Harbor – three Japanese carriers for three American – would threaten the whole enterprise. Yamamoto was no fool, however. He well understood the risk.

He also understood better than any other senior naval commander in the region what advances in aviation had done to the traditional plans of the American and Japanese navies. Yamamoto had no doubt that Japan’s ultimate goal was still a negotiated settlement with the United States. However, he also knew that the increases in range and striking power of carrier and landbased planes gave him the chance to do with aviation what the naval limitation agreements prevented him from doing with a surface fleet – to be in all the places that mattered in sufficient strength to keep the military initiative. To conquer both the resources needed by Japan and a buffer zone between Japan and the United States, Yamamoto needed to be able to “run free” for about six months. Then, his initial objectives in hand, he could adopt the traditional waiting game. There was really no other alternative, and Yamamoto threatened to resign if the General Staff did not set aside its reservations.

A similar understanding of the impact of long-range aviation was shaping the policies of the War Department in the United States. While the Washington agreements were in effect, the US Army was not allowed to improve or enlarge its Philippine defences. After Japan withdrew from the agreements in 1936, Congress declined to appropriate funds for Philippine fortifications because the United States was scheduled to grant independence to the islands in 1946 and because the Army and Navy did not think that scarce military appropriations should be diverted to what was regarded as a lost cause. However, in 1941 the Army’s B-17 four-engined bomber looked like just the mobile weapon which might make defense of the Philippines feasible. The heavy bombers had demonstrated that they could find and attack ships at sea, although under admittedly artificial conditions, and they had the range to reach Japanese airfields on Formosa. Secretary of War Henry L. Stimson and Gen. Douglas MacArthur (who had been recalled from retirement to lead US and Philippine troops) were convinced that land-based air power could take up the slack left by the transfer of American aircraft carriers to the Atlantic. In September, the first B-17s landed at Clark Field on the island of Luzon, near Manila. MacArthur hoped to have 300 of them by April 1942. When Pearl Harbor was attacked, he had 35.

It was too little, too late. On November 5, the Japanese set a date for the Pearl Harbor attack; on the 26th, the Pearl Harbor Striking Force left its anchorage in Etorofu, in the Kurile Islands. The Force, commanded by Vice Adm. Chuichi Nagumo, was built around the following carriers: Akagi (commissioned 1927 and later modernized), Kaga (1928, also modernized), Hiryu (1939), Soryu (1937), Shokaku, and Zuikaku (both commissioned 1941). These ships carried 423 aircraft, their maximum complement. Escorting them were the modernized battle cruisers Hiei and Kirishima, two heavy cruisers (Tone and Chikama), and a squadron of nine Kagero class destroyers. Patrolling ahead of the Striking Force were three I class submarines; supporting it were eight tankers and supply ships. Steaming at latitude 43 degrees North, the Force turned southeast on December 3, and headed for a point 275 miles (440km) north of Oahu.

Proceeding separately to Hawaiian waters (via the Marshalls) was the Advance Expeditionary Force: 27 large, long-range submarines – 11 with small aircraft and five carrying midget submarines for use inside Pearl Harbor. Each midget carried two torpedoes. The large submarines were to remain off Oahu after the initial attack; their job was to ambush any ships which the aircraft could not sink. The whole plan was bold. Adm. Nagumo did not assume that he could approach undetected; he had orders to attack even if detected in his final approach to Pearl Harbor. If US ships left the harbor during his approach, he was to find and sink them. He was not to assume complete surprise. The chance that the midget submarines and the aircraft of Nagumo’s force might not strike simultaneously was accepted as worth the risk. Soon after 6:00 A.M. on December 7, the six carriers turned into the wind and launched the first wave of 183 planes. Another 170 aircraft were launched in a second wave an hour later. Of the 353 planes sent against Pearl Harbor, 50 carried 40-centimeter armor-piercing shells converted to bombs for use against battleships, 40 carried torpedoes with 450-pound warheads, 50 were level bombers with 250-kilogram high explosive bombs, 80 were Zero fighters, and the rest were dive bombers.

At anchor in Pearl Harbor were 70 warships, including 8 battleships, 2 heavy cruisers, 6 light cruisers, 29 destroyers, and 5 submarines. There were also 24 auxiliaries, among them 10 destroyer and submarine tenders, 2 oilers, 3 repair ships, and a hospital ship. Adm. Kimmel and the Army commander in Hawaii, Maj. Gen. Walter Short, had already been sent several “war warnings” by their superiors in Washington. As recently as November 27, Chief of Naval Operations Stark had cautioned Kimmel, largely on the basis of intercepted Japanese diplomatic traffic, that Japan was expected to make an “aggressive move” in a matter of days. However, Stark had also said that the evidence “indicates an amphibious expedition against either the Philippines, Thai or Kra Peninsula or possibly Borneo”. As Stark’s warnings demonstrated, the cryptanalysts who had broken the Japanese diplomatic codes could chart the breakdown of relations between the United States and Japan, but they tended to produce vague and sometimes contradictory indications of a Japanese attack “somewhere” in the Pacific. Since October, the Navy’s signal intelligence station on the island of Corregidor in Manila Bay had been monitoring Japanese naval and merchant marine communications in ports such as Shanghai and in the South China Sea. The Japanese clearly were on the move, but the volume and sources of radio traffic pointed toward a southward advance, since Nagumo’s force maintained strict radio silence as it steamed eastward.