<P> The military history of the United States in World War II covers the war against Germany , Italy , Japan and 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 officially in the Quarantine Speech delivered by U.S. President Franklin D. Roosevelt in 1937 , while supplying Britain , the Soviet Union , and China with war material through the Lend - Lease Act which was signed into law on 11 March 1941 , as well as deploying the U.S. military to replace the British invasion forces in Iceland . In the Pacific Theater , there was unofficial early U.S. combat activity such as the Flying Tigers . The U.S. economic sanctions on Japan , as part of the effort to deter Japanese military aggression in Asia and the Pacific , was a major cause of the Japanese attack on Pearl Harbor . During the war , over 16 million Americans served in the United States Armed Forces , with 405,399 killed in action and 671,278 wounded . 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 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 . Highest priority went to the defeat of Germany in Europe , but first the war against Japan in the Pacific was more urgent after the sinking of the main battleship fleet at Pearl Harbor . </P>
<P> The military history of the United States in World War II covers the war against Germany , Italy , Japan and 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 officially in the Quarantine Speech delivered by U.S. President Franklin D. Roosevelt in 1937 , while supplying Britain , the Soviet Union , and China with war material through the Lend - Lease Act which was signed into law on 11 March 1941 , as well as deploying the U.S. military to replace the British invasion forces in Iceland . In the Pacific Theater , there was unofficial early U.S. combat activity such as the Flying Tigers . The U.S. economic sanctions on Japan , as part of the effort to deter Japanese military aggression in Asia and the Pacific , was a major cause of the Japanese attack on Pearl Harbor . During the war , over 16 million Americans served in the United States Armed Forces , with 405,399 killed in action and 671,278 wounded . 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 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 . Highest priority went to the defeat of Germany in Europe , but first the war against Japan in the Pacific was more urgent after the sinking of the main battleship fleet at Pearl Harbor . </P>
when did world war 2 begin for the united states
when did world war 2 begin for the united states
7 December 1941
7 December 1941