“That day ended isolationism.”

Senator Arthur Vandenberg describing the effect
of the Japanese attack on Pearl Harbor,
Hawaii, December 7, 1941

By late 1941 the United States and Japan had clashed over Japanese expansion in China and Indochina for over a decade. When the United States attempted to contain this aggression by imposing embargos on raw materials needed by the Japanese, the diplomatic situation deteriorated, and while negotiations continued, both sides began to prepare for war. The United States reinforced its Pacific bases in the Philippine Islands and at Pearl Harbor, Hawaii. The Japanese military began preparing for an attack on American, British, and Dutch holdings in Asia and the Pacific. When negotiations broke down in late November, these plans went into effect.

War came on Sunday, December 7. Just before 8 a.m., Japanese bombers and torpedo planes attacked the U.S. Pacific Fleet at Pearl Harbor. The surprise was complete, the devastation nearly so. Two waves of planes sank four U.S. battleships, damaged four others, and sank three cruisers and three destroyers. Almost 250 American aircraft were destroyed. There were more than 3,500 American casualties.

The next day President Roosevelt addressed a joint session of Congress. Pronouncing December 7, 1941, "a date which will live in infamy," the President asked that Congress declare war on Japan. Shortly afterward, Congress voted, with only 1 dissent, for a Declaration of War. On December 11 Germany and Italy declared war on the United States.