In the course of looking for information on a British ancestor, you may come across a situation where your ancestor married in a known locality, but you find no trace of his birth and parentage even after conducting a radius search of parishes as far as 20 miles or so. One answer may be that he was in the military and met his wife while he was stationed or undergoing training hundreds of miles away from where he was born and raised.
Military records are potentially a gold mine of genealogical information not found anywhere else. These records identify individuals who served or were eligible to serve in the military. Evidence that an ancestor served in the military can come from family records, biographies, census records, probates (wills), civil registration and church records. Sometimes all of these kinds of sources will need to be consulted before the right clue is found.
Service in the military was usually a lifetime career. The exception was militia service. Officers came from the upper classes while soldiers normally came from the poorer classes. A compulsory draft seldom was used, and then only by the militia. The army began as a permanent organization in 1660. Scottish regiments were added in 1707 with the union of the Kingdoms of England and Scotland. The soldiers in the regular army or navy could have entered military service through any number of stations located throughout the British Isles.
The first task in finding an ancestor in military records is to learn the name or number of the regiment he belonged to or the ship he served on. Once you know the regiment or ship, muster rolls may help you trace your ancestor’s career, age, and birthplace.
Muster rolls were used by army, navy, and militia. These records usually list each person assigned to a ship or regiment at the muster date, his age on joining, the date he joined, the place where he joined and sometimes information such as dependent list and birthplace. Army musters exist from 1760 to 1878. Navy musters cover 1667 to 1878.
There are many other military records, but the muster rolls are a great place to start.
For information on British military records, see the British Military Records article in the FamilySearch Research Wiki.