The American militia movement is a term used by law enforcement and security analysts to refer to a number of private organizations that include paramilitary or similar elements. These groups may refer to themselves as militia, unorganized militia, and constitutional militia. While groups such as the Posse Comitatus existed as early as the 1980s, the movement gained momentum after standoffs with government agents in the early 1990s. By the mid-1990s, such groups were active in all 50 US states, with membership estimated at between 20,000 and 60,000. The movement is most closely associated with the American right-wing, though other groups may range from the far right to the far left of the political spectrum.
Although the far-right patriot movement had long been a fringe factor in American politics, cultural factors paved the way for the wide-scale growth of the ideological militia movement. The catalysts came with the FBI's 1992 shootout with Randy Weaver at Ruby Ridge, and the 1993 Waco siege involving David Koresh and the Branch Davidians at Mt. Carmel in Waco, Texas. Critic Mark Pitcavage described the militia movement of the 1990s:
The militia movement is a right-wing movement that arose following controversial standoffs in the 1990s. It inherited paramilitary traditions of earlier groups, especially the conspiratorial, anti-government Posse Comitatus. The militia movement claims that militia groups are sanctioned by law but uncontrolled by government; in fact, they are designed to oppose a tyrannical government. The movement's ideology has led some adherents to commit criminal acts, including stockpiling illegal weapons and explosives and plotting to destroy buildings or assassinate public officials, as well as lesser confrontations.
During the 1990s public attention to the militia movement began to grow. The Oklahoma City bombing on April 19, 1995, the second anniversary of the Waco fire, drew nationwide attention to the militia movement as Timothy McVeigh was associated with the Michigan Militia, having possibly attended meetings before the attack.