H.R. 3545, An Act to authorize the President to increase temporarily the Military Establishment of the United States (Selective Service Act), April 28, 1917
Under the terms of the Selective Service Act of 1917, all males aged 21 to 30 were required to register for military service. Congress amended the law in 1918 to include all men 18 to 45. By the end of World War I, 2.8 million men had been drafted, and 2 million men had volunteered.
Records of the U.S. Senate, National Archives and Records Administration
Serving the Nation: The Draft
Less than six weeks after the United States entered World War I, Congress narrowly passed the Selective Service Act of 1917, giving the president power to draft men for war service. In 1940, after the German defeat of France and amid growing concerns about a possible German attack on the United States, Congress approved the country’s first-ever peacetime draft. The combined wartime drafts required nearly 13 million American men to register for wartime service.
If it were possible to raise a sufficient number of men by voluntary enlistment and they would come from all sections in equal percentages the selective service might not be necessary.
Speaker of the House Sam Rayburn of Texas, Constituent Correspondence, August 22, 1940