The United States entered World War II on the side of the Allies in response to the Japanese surprise attack on Pearl Harbor on December 7, 1941. President Franklin D. Roosevelt declared war on Japan the next day, and on December 11, Germany and Italy declared war on the United States, leading to their involvement in the European theater of the war as well. The United States had previously been providing aid and support to the Allies, but the attack on Pearl Harbor marked the direct involvement of the U.S. military in the war effort.
