There is no simple definition of a group qualifying as a militia, however the following general criteria can be used as a guideline: (1) a militia is a domestic organization with two or more members; (2) the organization must possess and use firearms; and (3) the organization must conduct or encourage paramilitary training.
Other terms used to describe militias are Patriots and Minutemen.
Officials at the FBI Academy in the US classify militia groups in the US within four categories, ranging from moderate groups who do not engage in criminal activity to radical cells which commit violent acts of terrorism. Issues which have served as motivating factors for the development of the militia movement in the US include gun control, the incidents at Ruby Ridge (1992) and Waco (1993), the Montana Freemen Standoff (1996) and the restriction of land use by federal agencies.
The majority of growth within the militia movement in the US occurred during the 1990s.