Militia refers to a body of citizens armed and trained by the state for military service apart from the regular armed forces. It is composed of physically fit civilians eligible by law for military service. It characterizes a military force recruited directly from civilians who would not otherwise be liable to serve in a state's regular armed forces. Usually the state imposes military obligation on the militia for the purposes of local or home defense and in case of emergencies. However, on occasions such militias have been employed abroad also.
It can also refer to unorganized military force drawn from within a civilian population and which has taken up arms. For example in modern Somalia the armed followers of different warlords have been characterized as militias.
The U.S Constitution recognizes a state’s rights to form a “well regulated militia.” The Second Amendment of the Bill of Rights provides: "A well regulated Militia, being necessary to the security of the free State, the right of the people to keep and bear Arms, shall not be infringed." The Supreme Court has historically defined the Second Amendment as giving states the right to maintain a militia separate from a federally controlled army. The Constitution also provides for the “calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions ; To provide for organizing, arming, and disciplining the Militia, and for governing such part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress.” (U.S. Constitution, art I, § 8, cl 15-16)
Examples of militia within US: