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:

lincoln presided over the union victory in the american civil war, which dominated his presidency. a former whig, lincoln ran on a political platform opposed to the expansion of slavery in the territories. his election served as the immediate impetus for the outbreak of the american civil war.

representative government

explanation:

american revolution: major events leading to declaration of independence. 1765-1766; tax introduced on legal documents to pay for defense of colonies. 1766: stamp act repealed after merchants protesting due to their goods not being bout and officials being threatened.

this act gave the federal government the power to relocate any native in the east to territory west of the mississippi river.