Here you go, (long post warning)
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There is a long history of militia in the United States, starting during the colonial era, with the colonial militias normally consisting of all adult white male citizens of a community, town, or local region. Colonial militia served vital roles in the French and Indian Wars and the United States Revolutionary War. Militia service shifted from colonial control to state controlled in the early years of the United States with the Second Continental Congress and the Articles of Confederation. With Constitutional Convention of 1787 and Article 1 Section 8 of the United States Constitution, the power to call up the state based milita was transfered to the federal congress. Two years later, with the Bill of Rights in 1789, the congress provided that the federal government could not infringe on the rights of the states to maintain the militia by ratification of the Second Amendment. A "well-regulated" militia means well-trained and well-organized. Thus, the term would not have been properly used to refer to an armed, unruly mob, but only to persons who behave in a responsible, law-enforcing mode, and who might act to control an armed, unruly mob as an "insurrection".
After the Civil War, state guard units composed of select militia were created. After 1903, the militia was divided into two groups, unorganized and organized. Organized units were created from portions of the former state guards and became state National Guard units. Some states later created State Defense Forces for assistance in local emergencies. Privately organized citizens-militias, not affiliated with any government organization, and usually formed by citizens suspicious of the activities and politics of Federal and state governments, blossomed in the mid 1990s.
A Constitutional Militia Movement consists of citizen private paramilitary militia groups who espouse strict construction of the U.S. Constitution according to the original understanding and intent of the Founding Fathers of the United States, especially in regard to the right to keep and bear arms (see Second Amendment to the United States Constitution). Constitutional Militias train in the proper and safe use of firearms, that they may be effective if called upon by the federal congress to uphold liberty, protect the people in times of crisis (i.e. disasters such as Hurricane Katrina), or to defend against invasion and terrorism. U.S. Constitution, Art. I Sec. 8 Cl. 15 & 16.
According to Title 10, USC, Section 311, all able bodied males between the ages of 17 and 45 (who are citizens or have declared their intent to become citizens) not serving in the armed forces or state national guard units are considered the unorganized militia, as well as all female citizens in state national guard units. Honorably discharged veterans of the armed forces regular components under the age of 64 may be considered as part of the unorganized militia pool, with congressional authorization to be enrolled for enlistment, warrant, or commission in national guard units until reaching that age.
"That the People have a right to keep and bear Arms; that a well regulated Militia, composed of the Body of the People, trained to arms, is the proper, natural, and safe Defense of a free state..." --George Mason, declaration of "the essential and unalienable Rights of the People," later adopted by the Virginia ratification convention, 1788.