militia, military organization composed of citizens enrolled and trained for service in times of national emergency. Its ranks may be filled either by enlistment or conscription. An early prototype was the national militia developed by Philip of Macedon. However, the modern concept of the militia as a defensive organization against invaders grew out of the Anglo-Saxon fyrd. The militiaman, in times of crisis, left his civilian duties and became a soldier until the emergency was over, when he returned to his civilian status. Militias persisted through the Middle Ages, especially in England, Italy, and Germany; after the rise of large standing armies they declined.

In America, however, militias survived. The Military Company of Massachusetts was the first militia organization in America and was followed by similar groups in the other colonies. Local control and voluntary service prevailed. Although the militia was valuable throughout the American Revolution, it proved undependable in the War of 1812. Therefore, no militia forces were used in the Mexican War. However, during the Civil War, when manpower needs were greater, both sides resorted to the use of militia. After World War I, state military units were established under the term National Guard. In other countries the militia is known generally as the special reserve or the territorial reserve.

In 1995 the bombing of Oklahoma City's federal building focused national attention on self-appointed "militias" or, as they often call themselves, "Patriots." These armed, typically rural and predominantly male organizations, many of which are in Western states, have a membership largely consisting of a mix of survivalists, white supremacists, gun-control opponents, "Christian Identity" adherents, and others adamantly opposed to most involvement of the federal government in the daily lives of U.S. citizens. Many in the militia movement were particularly angered by the FBI siege at Ruby Ridge, Idaho (1992), the destruction of the Branch Davidians at Waco, Tex. (1993), and the passage of the Brady Bill handgun control legislation (1993); these events spurred the further growth of the American militia movement in the 1990s.

Military organization of citizens with limited military training who are available for emergency service, usually for local defense. In many countries the militia is of ancient origin. The Anglo-Saxons required every able-bodied free male to serve. In colonial America it was the only defense against hostile Indians when regular British forces were not available. In the American Revolution the militia, called the Minutemen, provided the bulk of the American forces. Militias played a similar role in the War of 1812 and the American Civil War. State-controlled volunteer militias in the U.S. became the National Guard. British militia units, begun in the 16th century for home defense and answerable to the county sheriff or lord lieutenant, were absorbed into the regular army in the 20th century. Today various paramilitary organizations, from U.S. white supremacists to revolutionaries in the developing world, use the term militia to accentuate their populist origins.

Learn more about militia with a free trial on Britannica.com.