Japan's aggression in China, military alliance with Hitler, and proclamation of a "Greater East Asian Co-Prosperity Sphere" that included resource-rich Southeast Asia were major milestones along the road to war, but the proximate cause was Japan's occupation of southern French Indochina in July 1941, which placed Japanese forces in a position to grab Malaya, Singapore, and the Dutch East Indies. Japan's threatened conquest of Southeast Asia, which in turn would threaten Great Britain's ability to resist Nazi aggression in Europe, prompted the administration of Franklin D. Roosevelt to sanction Japan by imposing an embargo on U.S. oil exports upon which the Japanese economy was critically dependent. Yet the embargo, far from deterring further Japanese aggression, prompted a Tokyo decision to invade Southeast Asia. By mid-1941 Japanese leaders believed that war with the United States was inevitable and that it was imperative to seize the Dutch East Indies, which offered a substitute for dependency on American oil. The attack on Pearl Harbor was essentially a flanking raid in support of the main event, which was the conquest of Malaya, Singapore, the Indies, and the Philippines, Japan's decision for war rested on several assumptions, some realistic, others not. The first was that time was working against Japan--i.e., the longer they took to initiate war with the United States, the dimmer its prospects for success. The Japanese also assumed they had little chance of winning a protracted war with the United States but hoped they could force the Americans into a murderous, island-by-island slog across the Central and Southwestern Pacific that would eventually exhaust American will to fight on to total victory. The Japanese believed they were racially and spiritually superior to the Americans, whom they regarded as an effete, creature-comforted people divided by political factionalism and racial and class strife.
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The Japanese attack on Pearl Harbor, Hawaii, continues to perplex. American naval historian Samuel Eliot Morison called Tokyo's decision for war against the United States "a strategic imbecility." How, in mid-1941, could Japan, militarily mired in China and seriously considering an opportunity for war with the Soviet Union, even think about yet another war, this one against a distant country with a 10-fold industrial superiority? The United States was not only stronger; it lay beyond Japan's military reach. The United States could out-produce Japan in every category of armaments as well as build weapons, such as long-range bombers, that Japan could not; and though Japan could fight a war in East Asia and the Western Pacific, it could not threaten the American homeland. In attacking Pearl Harbor, Japan elected to fight a geographically limited war against an enemy capable of waging a total war against the Japanese home islands themselves.
Did the Japanese recognize the odds against them?
