The United States entered WWI in April of 1917. Less than a month later, the law below was passed by Congress:
The Selective Service Act of 1917: required registration of all men aged 21 through
30, was intended to supply the military and war-related industries, provided
exemptions for civil servants and clergy, exempted conscientious objectors from
combat, allowed no paid substitutes, 9.9 million men were registered by June and
687, 000 men were immediately inducted.
So many people were required so quickly because the US had previously: