The military history of the United States in World War II covers the victorious Allied war against the Axis Powers, starting with the 7 December 1941 attack on Pearl Harbor. During the first two years of World War II, the United States had maintained formal neutrality as made official in the Quarantine Speech delivered by US President Franklin D. Roosevelt in 1937, while supplying Britain, the Soviet Union, and China with war materiel through the Lend-Lease Act which was signed into law on 11 March 1941, as well as deploying the US military to replace the British forces stationed in Iceland. Following the "Greer incident" Roosevelt publicly confirmed the "shoot on sight" order on 11 September 1941, effectively declaring naval war on Germany and Italy in the Battle of the Atlantic.^[80] In the Pacific Theater, there was unofficial early US combat activity such as the Flying Tigers.

During the war some 16,112,566 Americans served in the United States Armed Forces, with 405,399 killed and 671,278 wounded.^[81] There were also 130,201 American prisoners of war, of whom 116,129 returned home after the war.^[82]

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 1943–45, where US forces, representing about a third of the Allied forces deployed, bogged down after Italy surrendered and the Germans took over. Finally the main invasion of France took place in June 1944, under General Dwight D. Eisenhower. Meanwhile, the US Army Air Forces and the British Royal Air Force engaged in the area bombardment of German cities and systematically targeted German transportation links and synthetic oil plants, as it knocked out what was left of the Luftwaffe post Battle of Britain in 1944. Being invaded from all sides, it became clear that Germany would lose the war. Berlin fell to the Soviets in May 1945, and with Adolf Hitler dead, the Germans surrendered.