What was the key to U. S. success in ww2?

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 1937 Quarantine Speech delivered by U. S. President Franklin D. Roosevelt on 5 October 1937 in Chicago, 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,