In May of 1917, the Selective Service Act was passed. It authorized the President to increase the national army through conscription. There were three draft registrations. The first on June 5, 1918, all men in the U.S. between the ages of 21 and 31 were to register with their local draft board. There was a second draft registration a year later for those who had turned 21 after the first draft registration with a supplemental registration on August 24, 1918 for those had turned 21 after June 5, 1917. The third and final registration was expanded to include men between 18 and 45 years of age on September 12, 1918.
The approximately 24 million draft registration cards (NARA Microfilm publication M1509) have been microfilmed and are indexed on a several sites; which encompasses almost 25% of the population of the U.S. in 1918. Indexed and searchable copies of the record group are available on Ancestry, Family Search, and Fold3.
How are these records useful in your family history research?
They contain details about the individual that may not be found anywhere else: date and place of birth, physical description, street address, employer, name and residence of closest relative, and signature.
As with all records- look on the back and the record before and after!