Henry L. Stimson in the context of "Stimson Doctrine"

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 Japan and exited it with the surrender of Japan on 2 September 1945. During the first two years of World War II, the U.S. maintained formal neutrality, which was officially announced in the Quarantine Speech delivered by President Franklin D. Roosevelt in 1937. While officially neutral, the U.S. 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 U.S. 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 407,316 killed and 671,278 wounded. According to the US Department of Defense, of the 407,316 dead, about 250,000 were killed in the European theater, the remaining 160,000 died in the Pacific War. There were also 130,201 American prisoners of war, of whom 116,129 returned home after the war. 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 U.S. Army, commanded by General George C. Marshall and the Army Air Forces under General Henry H. Arnold. The U.S. Navy, led by Secretary of the Navy Frank Knox and Admiral Ernest J. King, proved more autonomous. Overall priorities were set by Roosevelt and the Joint Chiefs of Staff, chaired by William D. Leahy. The defeat of the Nazis was the U.S.'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.