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World War II: 1939–1952

Attack on Pearl Harbor and Entrance of the United States into World War II

Japan's decision to bomb the U.S. naval base at Pearl Harbor, Hawaii, on December 7, 1941, triggered the United States' entry into World War II.

The United States was a trading partner with Japan even while negotiating Lend-Lease—the loan of equipment and supplies to vital European allies. Japan had invaded China in 1937 and allied itself with Germany in 1940. Many Americans were uneasy about providing fuel and steel to an Axis power. In 1941 the United States banned the sale of such supplies to Japan. A freeze—or hold—was also put on Japanese assets in the United States. Officials hoped these actions would pressure Japan into exiting China. Instead, Japan secretly decided to declare war. On the morning of December 7, 1941, Japan ended diplomatic relations with the United States. An attack was expected, but its location was unknown to the Americans.

Within minutes of ending diplomatic relations, Japan dispatched a fleet of bombers and fighter planes from aircraft carriers located in the Pacific. The planes headed for Pearl Harbor on the Hawaiian island of Oahu—the main base of the U.S. Pacific Fleet. Beginning around 8:00 a.m., Japanese forces attacked the base—dropping bombs and torpedoes on ships anchored in the harbor and aircraft parked at the airfields. Within two hours, the Japanese had destroyed most of the U.S. fleet, including nearly half of its aircraft and seven of its eight battleships. They killed over 2,400 and wounded nearly 1,200 American servicemembers and civilians. That same day, Japanese forces invaded the Philippines, Guam, Midway Island, and Hong Kong.
"While the Japanese seriously damaged the U.S. Pacific Fleet, they did not destroy essential fuel depots or the aircraft carriers that were at sea when the attack occurred.
Credit: Courtesy of the U.S. Naval Institute Photograph Collection. U.S. Naval History and Heritage Command Photograph.
The next day, Americans clustered around radios heard the Day of Infamy speech, President Roosevelt's call to Congress to declare war on Japan. In the Day of Infamy speech, Roosevelt announced, "Yesterday, December 7, 1941—a date which will live in infamy—the United States was suddenly and deliberately attacked by ... the Empire of Japan." A near unanimous vote by Congress marked the United States' entry into war with Japan. Only Montana representative Jeannette Rankin, who had voted also against U.S. entry into World War I, objected. On December 11, Germany and Italy declared war on the United States. The Americans had now officially entered World War II.

The attack on Pearl Harbor had two effects. First, it unified the American public and ended most support for neutrality. Second, it triggered a massive war effort. Although the Pearl Harbor assault significantly weakened the U.S. fleet, over the next few years the United States would build the world's most powerful military.