Military Sources
Military records are complex. They generally must be searched on a war by war basis.
Civil War Soldiers and Sailors System
World War II Enlistment Records. Article in the National Archives periodical, Prologue: The World War II Army Enlistment Records File and Access to Archival Databases
The National Archives includes military records.
Ancestry Learning Center Research Guides: scroll down to Military
Reference Books
How to locate anyone who is or has been in the military: Armed Forces locator directory, by Richard S. Johnson. 1995 ed. San Antonio, TX: MIE Publishing, 1995, 271 p.
Civil War Research Guide: a guide for researching your Civil War ancestor, by Stephen McManus, Thomas Churchill, Donald Thompson. Mechanicsburg, PA: Stackpole Books, 2003, 115 p. Online access available via Internet Archive.