In 1940 the American Army was composed of three different structures: The Regulars (243,095 Men), The National Guard (226,837 Men) and the Organised Reserve (104,228 Men). By June 1941 the regulars had rose to 280,000 and this total rose again to 375,000 in little over a week. Two months later the Regular Army and National Guard amounted to 606,915. When the US war machine got fully mobilised the army grew from 1,657,157 in late 1941 to 5,400,888 in less than two years. In total the United States army formed 91 Divisions.
The US Marines formed six Divisions with the 1st and 2nd divisions being activated in February 1941. The 3rd and 4th followed in 1942 and the 5th and 6th in 1943 and 1944 respectively. In total the United States Marine Corps grew from 27,925 in 1940 to 485,053 in 1945.