The three young men showing their registration cards were among the millions of men who registered for the draft after the United States entered World War I in 1917. When war broke out, the U.S. Army’s strength was just over 200,000 men. The Selective Service Act of May 1917 increased the size of the Army for the duration of the war. Men between the ages of 18 and 45 were eligible for the draft; more than 2.7 million were drafted during those years.
National Archives, Records of the War Department General and Special Staffs