The Pearl Harbor attack
The Japanese surprise attack on Pearl Harbor began just before 8 a.m. local time on December 7, 1941. For over an hour, Japanese aircraft attacked the naval base, destroying U.S. naval vessels and U.S. aircraft. 2,403 Americans, including 68 civilians, died in the attack.
On December 8, President Roosevelt addressed a joint session of Congress: Yesterday, December 7, 1941—a date which will live in infamy—the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan.
In his address, Roosevelt asked Congress to declare war against Japan, which it did that day. Three days later, Japan's allies Germany and Italy declared war on the United States, and Congress then declared war on Germany and Italy. Americans now supported U.S. entry into the war.
Motive for the attack
Japan attacked Pearl Harbor because the United States cut off its oil exports to Japan, which constituted 80% of Japanese oil supply. Japan's navy needed this oil for the war. Japan hoped the attack would cripple the U.S. Pacific fleet. With the British, French, and Dutch at war in Europe, the Japanese believed the European powers would be unable to defend their Asian colonial holdings and that Japan would be able to take control of them as well.
Forewarnings about the attack
The attack on Pearl Harbor took the United States by surprise. It had decoded a Japanese message about the attack, but bad weather prevented transmission of Gen. Marshall's message to Pearl Harbor warning of imminent attack. The United States also had known that a Japanese attack was likely somewhere in the Pacific, but they thought that the Japanese would attack a U.S. target closer to Japan than Hawaii.
After Pearl Harbor, the United States rapidly mobilized for World War II.
Source: Pearl Harbor
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