Why did Japan attack Pearl Harbor?
Japan attacked Pearl Harbor on December 7, 1941, because American oil and trade embargoes threatened to cripple Japan's military machine and imperial ambitions in Asia. Japanese leaders calculated that a devastating surprise attack on the U.S. Pacific Fleet would buy time to conquer resource-rich Southeast Asia before America could rebuild and respond.
Japan's attack on Pearl Harbor was not an act of irrational aggression but a calculated — if ultimately catastrophic — strategic gamble by a militarist government that saw war with the United States as inevitable and sought to fight it on the most favorable terms possible.
The roots lay in Japan's imperial expansion. Since the Meiji Restoration, Japan had pursued a policy of industrial modernization and military expansion. Victories over China (1895) and Russia (1905) had established Japan as a regional power. The invasion of Manchuria in 1931 and the full-scale war against China beginning in 1937 demonstrated Japan's determination to create a vast Asian empire — what Japanese propagandists called the 'Greater East Asia Co-Prosperity Sphere.'
The critical problem was resources. Japan's home islands lacked the oil, rubber, tin, and iron needed to sustain its military machine. As long as these materials could be imported, expansion was possible. But Japan's war in China alarmed the Western powers, particularly the United States. In 1940 and 1941, the U.S. imposed increasingly severe economic sanctions, culminating in a complete embargo on oil exports to Japan in July 1941. Since Japan imported roughly 80% of its oil from the United States, this was effectively an ultimatum: withdraw from China and Southeast Asia, or face economic strangulation.
Japan's military leaders refused to withdraw. They calculated that the oil-rich Dutch East Indies (modern Indonesia) could replace American supplies, but seizing them would mean war with Britain and the Netherlands — and inevitably with the United States. The plan, devised by Admiral Yamamoto Isoroku, was to destroy the American Pacific Fleet in a surprise attack, then rapidly conquer Southeast Asia and establish a defensive perimeter so strong that America would eventually negotiate a peace rather than fight a long war across the Pacific.
The attack on December 7, 1941, was tactically successful — it sank or damaged eight battleships and killed over 2,400 Americans. But it was strategically disastrous. The aircraft carriers, which were at sea during the attack, survived — and carriers, not battleships, would prove decisive in the Pacific War. More importantly, the attack united American public opinion behind the war effort with a fury that Japanese planners had not anticipated. Admiral Yamamoto reportedly said, 'I fear all we have done is to awaken a sleeping giant and fill him with a terrible resolve.' He was right.