The US Army of World War II was created from a tiny antebellum army in the space of three years. On 30 June 1939 the Regular Army numbered 187,893 men, including 22,387 in the Army Air Corps. On the same date the National Guard totaled 199,491 men.

On 7 December 1941 the Army consisted of 1,685,403 men (including 275,889 in the Air Corps) in 29 infantry, five armor, and two cavalry divisions.

Over the following three and a half years the Army expanded to 8,291,336 men in 89 divisions: sixty-six infantry, five airborne, sixteen armored, one cavalry, and one mountain infantry.