The United States is generally considered to have entered the conflict with the 7 December surprise attack on Pearl Harbor by the Empire of Japan and exited it with the 2 September surrender of Japan. Roosevelt in Following the 04 September Greer incident involving a German submarine, Roosevelt publicly confirmed a "shoot on sight" order on 11 September, effectively declaring naval war on Germany and Italy in the Battle of the Atlantic.
During the war, some 16,, Americans served in the United States Armed Forces , with , killed and , wounded. The defeat of the Nazis was the US's official highest priority per its agreement with Britain , however in practice the US devoted more resources to the Pacific than Europe and Africa until Admiral King put Admiral Chester W.
Nimitz , based in Hawaii, in charge of the Pacific War against Japan. The Imperial Japanese Navy had the advantage, taking the Philippines as well as British and Dutch possessions, and threatening Australia but in June , its main carriers were sunk during the Battle of Midway , and the Americans seized the initiative.
The Pacific War became one of island hopping , so as to move air bases closer and closer to Japan. With its merchant fleet sunk by American submarines, Japan ran short of aviation gasoline and fuel oil, as the US Navy in June captured islands within bombing range of the Japanese home islands. Strategic bombing directed by General Curtis Lemay destroyed all the major Japanese cities, as the US captured Okinawa after heavy losses in spring With the atomic bombings of Hiroshima and Nagasaki , the Soviet invasion of Manchuria , and the imminent invasion of the home islands , Japan surrendered.
The war in Europe involved aid to Britain, her allies, and the Soviet Union, with the US supplying munitions until it could ready an invasion force. US forces were first tested to a limited degree in the North African Campaign and then employed more significantly with British Forces in Italy in —45 , where US forces, representing about a third of the Allied forces deployed, bogged down after Italy surrendered and the Germans took over.
