The United States entered World War II because of Japan's attack on Pearl Harbor on the morning of Dec. 7, 1941. Previously, the United States had been neutral and wanted to avoid further involvement in European conflict despite financially aiding the United Kingdom with the Lend-Lease program and similar efforts.
The Japanese Navy, under the direction of Vice Adm. Chuichi Nagumo, launched a surprise attack on Pearl Harbor in Hawaii with the help of a plan drawn up by Adm. Irosoku Yamamoto. It was intended to cripple the naval capabilities of the United States.
Prior to Pearl Harbor, Japan had been expanding its territories in the Pacific and feared the United States would interfere. On top of the existing oil embargo against Japan, the presence of the U.S. Navy in the Pacific would have crippled Japan's ability to procure natural resources such as oil, rubber and tin. Therefore, Japan sent a force of dive bombers, fighters and aircraft carriers as well as submarines to attack the naval base early in the morning. The United States lost eight battleships, more than 150 fighter planes and more than 2,000 military personnel. However, the American aircraft carriers were untouched, enabling a quick rebuilding of the forces.