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.
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.