The reason why my father found himself back in the Army (with a 9-months pregnant wife) in September, 1942, after being discharged from the Army in September of 1941:
After Pearl Harbor the STSA [Selective Training and Service Act] was further amended (December 19, 1941), extending the term of service to the duration of the war plus six months and requiring the registration of all men 18 to 64 years of age. During World War II, 49 million men were registered, 36 million classified, and 10 million inducted. 18 and 19 year olds were made liable for induction on November 13, 1942. By late 1942, the Selective Service System moved away from a national lottery to administrative selection by its more than 6,000 local boards.
On December 5, 1942, presidential Executive Order 9279 made it so that all men from the ages of 18 to 37 could not voluntarily enlist for the duration of the war, providing protection for the nation's home front manpower pool. The Navy and Marine Corps began procuring their personnel through the Selective Service System in early 1943. The Navy and Marine Corps enlisted inductees and volunteers under the same service agreements, but with different service obligations, while the Army placed wartime inductees and volunteers into a special service component known as the Army of the United States, commonly known as the "AUS"; service commitments were set at the length of the war plus six months.