A military force composed of citizens, enlisted or conscripted in times of emergency, usually for local defence. In England, it developed during the Middle Ages from the Anglo-Saxon fyrd. Its forces were usually raised by impressment (forcible recruiting), and until the 16th century it was supervised by the local sheriff. Then the Lord Lieutenants were given the responsibility until the later 19th century.

Modern militias fulfil a variety of roles. Although generally restricted to infantry or light-armoured roles on land, in certain cases (for example, the US National Guard), they are deployed in air and coastal defence. Such countries as Israel, Switzerland, the former Yugoslavia, and Sweden have based much of their defence planning on the use of militia units, and people’s militias were a feature of former communist states, such as East Germany and Czechoslovakia. Militia forces can also play an important role in internal conflicts, where they may be raised and controlled by factions; in such cases, the dividing line between militia and guerrilla forces can be blurred.