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:

use wikipedia, i think they have this.

during the debate over his landmark energy policy proposal, president jimmy carter failed to consult one of the most powerful senators, louisiana’s russell long, who would have a big say over whether the legislation moved through the upper chamber. this was classic carter. with a strong distaste for congressional politics, the president simply refused to do what most other presidents would consider routine, such as courting key senators. long became so irate with the president that when he was finally invited to a white house dinner, he stood up to ask a question and, before doing so, proceeded to sarcastically introduce himself: “i’m russell long, chairman of the senate finance committee.”

this interaction is a revealing moment in stuart e. eizenstat’s fascinating new history of carter’s presidency. the former peanut farmer from georgia was a president who, according to conventional wisdom, didn’t care much for party politics; nonetheless, in eizenstat’s view, carter’s legacy on domestic and foreign policy has been vastly underappreciated. the author paints carter as a “bold and determined” leader who bravely attacked the challenges of his time. eizenstat goes so far as to argue that carter might very well be the most successful one-term president in american history.