What was the Selective Service Act, and why was it passed?

On May 18, 1917, Congress passed the Selective Service Act, which authorized the Federal Government to temporarily expand the military through conscription. The act eventually required all men between the ages of 21 to 45 to register for military service. Under the act, approximately 24 million men registered for the draft. Of the total U.S. troops sent to Europe, 2.8 million men had been drafted, and 2 million men had volunteered. To commemorate this anniversary, the draft registration cards Irving Berlin, Al Capone, Duke Ellington, Marcus Garvey, Harry Houdini, Fiorello LaGuardia, Norman Rockwell, and Babe Ruth are on exhibit.