When war broke out in Europe in September 1939, the Japanese, despite a series of victorious battles, had still not brought their war in China to an end: on the one hand, the Japanese strategists had made no plans to cope with the guerrilla warfare pursued by the Chinese; on the other, the Japanese commanders in the field often disregarded the orders of the supreme command at the Imperial headquarters and occupied more Chinese territory than they had been ordered to take. Half of the Japanese Army was thus still tied down in China when the commitment of Great Britain and France to war against Germany opened up the prospect of wider conquests for Japan in Southeast Asia and in the Pacific. Japan’s military ventures in China proper were consequently restricted rather more severely henceforth.

The German victories over the Netherlands and France in the summer of 1940 further encouraged the Japanese premier, Prince Konoe, to look southward at those defeated powers’ colonies and also, of course, at the British and U.S. positions in the Far East. The island archipelago of the Dutch East Indies (now Indonesia) along with French Indochina and British-held Malaya contained raw materials (tin, rubber, petroleum) that were essential to Japan’s industrial economy, and if Japan could seize these regions and incorporate them into the empire, it could make itself virtually self-sufficient economically and thus become the dominant power in the Pacific Ocean. Since Great Britain, single-handedly, was confronting the might of the Axis in Europe, the Japanese strategists had to reckon, primarily, with the opposition of the United States to their plans for territorial aggrandizement. When Japanese troops entered northern Indochina in September 1940 (in pursuance of an agreement extorted in August from the Vichy government of France), the United States uttered a protest. Germany and Italy, by contrast, recognized Japan as the leading power in the Far East by concluding with it the Tripartite, or Axis, Pact of September 27, 1940: negotiated by Japanese foreign minister Matsuoka Yosuke, the pact pledged its signatories to come to one another’s help in the event of an attack “by a power not already engaged in war.” Japan also concluded a neutrality pact with the U.S.S.R. on April 13, 1941.

On July 2, 1941, the Imperial Conference decided to press the Japanese advance southward even at the risk of war with Great Britain and the United States; and this policy was pursued even when Matsuoka was relieved of office a fortnight later. On July 26, in pursuance of a new agreement with Vichy France, Japanese forces began to occupy bases in southern Indochina.

This time the United States reacted vigorously, not only freezing Japanese assets under U.S. control but also imposing an embargo on supplies of oil to Japan. Dismay at the embargo drove the Japanese naval command, which had hitherto been more moderate than the army, into collusion with the army’s extremism. When negotiations with the Dutch of Indonesia for an alternative supply of oil produced no satisfaction, the Imperial Conference on September 6, at the high command’s insistence, decided that war must be undertaken against the United States and Great Britain unless an understanding with the United States could be reached in a few weeks’ time.

General Tōjō Hideki, who succeeded Konoe as premier in mid-October 1941, continued the already desperate talks. The United States, however, persisted in making demands that Japan could not concede: renunciation of the Tripartite Pact (which would have left Japan diplomatically isolated); the withdrawal of Japanese troops from China and from Southeast Asia (a humiliating retreat from an overt commitment of four years’ standing); and an open-door regime for trade in China. When Cordell Hull, the U.S. secretary of state, on November 26, 1941, sent an abrupt note to the Japanese bluntly requiring them to evacuate China and Indochina and to recognize no Chinese regime other than that of Chiang Kai-shek, the Japanese could see no point in continuing the talks. (See Sidebar: Pearl Harbor and the “Back Door to War” Theory.)

Since peace with the United States seemed impossible, Japan set in motion its plans for war, which would now necessarily be waged not only against the United States but also against Great Britain (the existing war effort of which depended on U.S. support and the Far Eastern colonies of which lay within the orbit of the projected Japanese expansion) and against the Dutch East Indies (the oil of which was essential to Japanese enterprises, even apart from geopolitical considerations).

The evolving Japanese military strategy was based on the peculiar geography of the Pacific Ocean and on the relative weakness and unpreparedness of the Allied military presence in that ocean. The western half of the Pacific is dotted with many islands, large and small, while the eastern half of the ocean is, with the exception of the Hawaiian Islands, almost devoid of landmasses (and hence of usable bases). The British, French, American, and Dutch military forces in the entire Pacific region west of Hawaii amounted to only about 350,000 troops, most of them lacking combat experience and being of disparate nationalities. Allied air power in the Pacific was weak and consisted mostly of obsolete planes. If the Japanese, with their large, well-equipped armies that had been battle-hardened in China, could quickly launch coordinated attacks from their existing bases on certain Japanese-mandated Pacific islands, on Formosa (Taiwan), and from Japan itself, they could overwhelm the Allied forces, overrun the entire western Pacific Ocean as well as Southeast Asia, and then develop those areas’ resources to their own military-industrial advantage. If successful in their campaigns, the Japanese planned to establish a strongly fortified defensive perimeter extending from Burma in the west to the southern rim of the Dutch East Indies and northern New Guinea in the south and sweeping around to the Gilbert and Marshall islands in the southeast and east. The Japanese believed that any American and British counteroffensives against this perimeter could be repelled, after which those nations would eventually seek a negotiated peace that would allow Japan to keep her newly won empire.

Until the end of 1940 the Japanese strategists had assumed that any new war to be waged would be against a single enemy. When it became clear, in 1941, that the British and the Dutch as well as the Americans must be attacked, a new and daring war plan was successfully sponsored by the commander in chief of the Combined Fleet, Admiral Yamamoto Isoroku.

Yamamoto’s plan prescribed two operations, together involving the whole strength of his navy, which was composed of the following ships: 10 battleships, six regular aircraft carriers, four auxiliary carriers, 18 heavy cruisers, 20 light cruisers, 112 destroyers, 65 submarines, and 2,274 combat planes. The first operation, to which all six regular aircraft carriers, two battleships, three cruisers, and 11 destroyers were allocated, was to be a surprise attack, scheduled for December 7 (December 8 by Japanese time), on the main U.S. Pacific Fleet in its base at Pearl Harbor in the Hawaiian Islands. The rest of the Japanese Navy was to support the army in the “Southern Operation”: 11 infantry divisions and seven tank regiments, assisted by 795 combat planes, were to undertake two drives, one from Formosa through the Philippines, the other from French Indochina and Hainan Island through Malaya, so as to converge on the Dutch East Indies, with a view to the capture of Java as the culmination of a campaign of 150 days—during which, moreover, Wake Island, Guam, the Gilbert Islands, and Burma should also have been secured as outer bastions, besides Hong Kong.