The military history of the United States during World War II covers the nation's role as one of the major Allies in their victory over the Axis Powers. The United States is generally considered to have entered the conflict with the 7 December 1941 surprise attack on Pearl Harbor by the Empire of Japan and exited it with the 2 September 1945 surrender of Japan. During the first two years of World War II, the US maintained formal neutrality, which was officially announced in the Quarantine Speech delivered by US President Franklin D. Roosevelt in 1937. While officially neutral, the US supplied Britain, the Soviet Union, and China with war materiel through the Lend-Lease Act signed into law on 11 March 1941, and deployed the US military to replace the British forces stationed in Iceland. Following the 4 September 1941 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. 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 291,557 killed and 671,278 wounded.[2] There were also 130,201 American prisoners of war, of whom 116,129 returned home after the war.[3] Key civilian advisors to President Roosevelt included Secretary of War Henry L. Stimson, who mobilized the nation's industries and induction centers to supply the Army, commanded by General George Marshall and the Army Air Forces under General Hap Arnold. The Navy, led by Secretary of the Navy Frank Knox and Admiral Ernest King, proved more autonomous. Overall priorities were set by Roosevelt and the Joint Chiefs of Staff, chaired by William Leahy. 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 1944.
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