On September 16, 1940, President Franklin Roosevelt signed the Selective Training and Service Act, and the United States had a peacetime draft for the first time in its history. Every man between 21 and 35 was required to register, about 16 million of them on the first registration day in October, and the first lottery drew 800,000 names.
The bill stalled for months. Germany had taken France in June, and the fall of France is what moved opinion, but the isolationist case was still strong and the 1940 election was three months away. The compromise was the Burke-Wadsworth Act: a year of training, service limited to the Western Hemisphere, no draftees sent abroad without Congress, and a quota system run by local boards of civilians rather than by the War Department. Those boards decided who was deferred, which meant they also decided who went.
What the act did in practice was build the army before the country was in the war. The Regular Army in 1940 had about 190,000 men and ranked behind Portugal's. Roosevelt asked for 1.2 million. The draft plus the National Guard call-up got the army to roughly 1.6 million by the middle of 1941, and the training camps got built, and the equipment contracts got placed, while the argument about whether to fight at all was still going on.
It nearly lapsed. In August 1941 the House renewed service beyond twelve months by 203 votes to 202, one vote, after a summer of picketing by the mothers of drafted men. Then Pearl Harbor settled the question, and the machinery was already there: 10 million men were eventually inducted through the same local boards.
The draft also forced changes the army had resisted. Black men were drafted at the same rate as white men, which produced the pressure that led to the Tuskegee air units and to separate but real combat formations, though segregation held until Truman's executive order in 1948. Conscription itself ran, with one break in 1947 and 1948, until 1973. Selective Service registration continues, and the last men required to register were born in 2007 or later.