Most people have a relative or ancestor who either served in the military or fought during a specific war. Many researchers are unaware, however, that a significant distinction exists between volunteer soldiers and Regular soldiers, and that the two types of service are documented differently. Volunteers (citizen soldiers) were enlisted to serve during specific wars or national crises; hence, the National Archives has records of volunteers who fought in all the major conflicts from the Revolutionary War to the Philippine Insurrection of 1899-1902. Regular soldiers, by contrast, were the professional or career soldiers who served during peace and war. They also fought in all of the major wars from 1775 to 1902, but their service was continuous; their units carried on in peacetime while the volunteer regiments disbanded.
Volunteer soldiers are documented in the Compiled Military Service Records (CMSRs), which contain all of their enlistment (muster-in) and discharge (muster-out) information. The War Department recorded the service of Regular soldiers in the Register of Enlistments (ARC ID 575272), which also showed the beginning and end of their terms of service. More information about both types of military service records is available on the Archival Research Catalog (ARC).
In the twentieth century and beyond, volunteer or citizen soldiers became a thing of the past (at least in the old sense) as the U.S. military conducted all of the fighting in America’s major wars (although you could now make a case for the National guardsmen who have been called to serve in Iraq and Afghanistan). The records of twentieth-century soldiers are probably a good subject for a separate blog!