Arm yourself with these resources for enlistments, muster rolls, discharge papers and other military-service records.
Chances are excellent that someone in your family tree was a soldier, sailor, airman, Marine, guardsman or state militiaman. Even if you don't have irrefutable evidence of an ancestor's military service, make a timeline of wars he lived through — it doesn't hurt to check for records if a man's age made him eligible to enlist. And don't assume your female ancestors didn't leave records: They may have nursed the injured in units such as the WWII Women's Army Auxiliary Corps, or applied for their veteran husbands' pension benefits.
Since military service was so common, you'll find lots of online resources — everything from huge, government-sponsored databases to a tiny town's WWI casualty list. Military records even encompass some who weren't in the armed forces: After the Selective Service Act of 1917, 24 million men aged 18 to 45 filled out WWI draft registration cards between June 1917 and September 1918. Even if he didn't enlist, your ancestor's card is probably on microfilm at the National Archives and Records Administration (NARA) <www.archives.gov>; some of the Web sites in this section have partial indexes.