Need some help finding military service records for your ancestors? Or finding out whether they served?

First, almost all adult men in the United States in 1917-1918 were registered for the World War I draft (1917-1918). Registration proceeded regardless of citizenship status, and those who served gained citizenship by serving. These draft cards are online for free at FamilySearch here.

Next, almost all adult men in the United States in 1940 were registered for the World War II draft. Draft records for men born before 1897 have been released by the National Archives and are online for free at FamilySearch here.

Next, to find which of those men actually served, the most comprehensive collection of available U.S. military records is held at Fold3. This website is organized by conflict, from the Revolutionary War to the war in Afghanistan. There is a membership fee to register to see records. OR, you can access Fold3 for free at a FamilySearch Center location near you.

A nice guide to draft records, along with explanations of military service records, is summarized on the website now known as American Ancestors, here.

Lists of US military records, organized by state, are listed on the FamilySearch Research Wiki page here. What information genealogists need that can be found on those records is described in general here.