Faction: The U.S. Army Before the war had started the US army was a small professional force. When Germany overran Poland within a matter of weeks in 1939, a greater urgency was put on building up the numbers in the Army already envisioned by the Rainbow plan. In 1940 the United States introduced selective conscription requiring registration of all men between 21 and 35 and by 1941 the US Army had grown to 1,500,000 men.