<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 materiel 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 . </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 materiel 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 . </P>
when did the us join world war ii
when did the us join world war ii
1941
1941