Three kinds of records, three kinds of evidence
Military genealogy splits into service records (who enlisted when, and where they served), pension files (what happened after), and bounty-land warrants (land granted in exchange for service). A soldier typically generated at least two of the three; pre-20th-century pensions are unusually rich because pension applicants had to prove identity, service, and continuing need with detailed affidavits.
Revolutionary War pension files run 50 to 200 pages and often include full narrative accounts of campaigns, names of officers, and — when a widow was claiming the pension — a marriage certificate or church record.
Reading muster rolls and service files
Muster rolls record an enlisted man's presence at each pay period. A row reads left to right as name, rank, unit, period covered, and remarks. Common remarks: Des. (deserted), K. (killed), Sk. (sick), W. (wounded), D.c. (discharged), P.o.W. (prisoner of war). Rank abbreviations follow period conventions — Pvt., Corp., Sgt., Lt., Capt. — and promotions appear as entries across successive muster periods.
Where to find them
The National Archives holds the authoritative originals. Revolutionary and Civil War pension files are substantially digitized on Fold3 and at FamilySearch. World War I draft registration cards — one for nearly every US man aged 18 to 45 in 1917–1918 — are universally accessible online and carry place of birth and next of kin. Unit histories, published by veterans' associations in the late 19th century, place a named soldier in a specific engagement when the service record is thin.