On May 18, 1917, the United States Congress passed the Selective Service Act, creating a draft to supplement the US troops during World War I.

At the time of World War I, the U.S. Army was tiny compared with the mobilized armies of the European powers. As late as 1914, the federal army was under 100,000, while the National Guard (the organized militias of the states) numbered around 120,000. By 1916, it had become clear that any participation by the United States in WWI would require a far larger army. President Woodrow Wilson at first wished to use only volunteers to supply the troops needed to fight, but it soon became clear that this would be impossible, as three weeks after war was declared, only 32,000 men had volunteered for service. Upon the recommendation of the Secretary of War, Wilson approved an act to create a draft.

The Act itself was drafted by Captain Hugh Johnson. It authorized the federal government to raise a national army numbering in the hundreds of thousands. All males aged 21 to 30 were required to register for military service. The law was later amended to include all men ages 18 to 45.

The main difference between this draft and earlier drafts (such as the Civil War draft) was that men could not hire a substitute to take their place if they were drafted. As only very rich people could afford to hire a substitute, the lower class tended to take the most casualties during fighting. The Selective Service Act contained language specifically prohibiting people from hiring substitutes.

By the end of World War I, around 24 million men had registered, and some 2.8 million had been drafted. In fact, more than half of the almost 4.8 million Americans who served in the armed forces during WWI were drafted. The act was cancelled with the end of the war in 1918.