United States entered World War II

The results of the attack on Pearl Harbor were many and significant.

Prior to the attack, public opinion in the United States had not been unanimous. When polled in January 1940, 60% of Americans were in favor of helping the United Kingdom in the war. A majority of Americans believed that the safety of the United States was contingent on the UK winning the war, and an even larger majority believed that the UK would lose the war if the United States stopped sending war materials to the United Kingdom. Despite this, the same poll reported that 88% of Americans would not support entering the war against Germany and Italy.

Americans were more unsure on the prospect of conflict with Japan around the same timeframe. In a February Gallup poll, a majority of Americans believed that the United States should intervene in Japan's conquest of the Dutch East Indies and Singapore. However, in the same poll, only 39% supported going to war with Japan, while 46% opposed the prospect.

Public support for assisting the United Kingdom rose through 1940, reaching about 65% by May 1941. However, opinions on the prospect of becoming involved against Germany and Italy still remained quite high with 80% disapproval.

On December 7, 1941, the Japanese launched a surprise attack on the U.S. naval base at Pearl Harbor. After two hours of bombing, 18 U.S. ships were sunk or damaged, 188 U.S. aircraft were destroyed, and 2,403 men were killed.

The day after the attack, President Franklin D. Roosevelt addressed a joint session of the 77th United States Congress. Roosevelt called December 7 "a date which will live in infamy". Congress declared war on the Empire of Japan amid outrage at the attack, the deaths of thousands of Americans, and the late delivery of the note from the Japanese government breaking off relations with the U.S. government. Pacifist Representative Jeannette Rankin, a Republican from Montana, cast the only dissenting vote. Roosevelt signed the declaration of war later the same day. Continuing to intensify its military mobilization, the U.S. government finished converting to a war economy, a process begun by provision of weapons and supplies to the Soviet Union and the British Empire. Japanese Americans from the West Coast were sent to internment camps for the duration of the war.

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