In general, a militia is the entire able-bodied population of a nation that can be called upon to defend itself against an enemy. It is distinguished from the organized military forces of that nation. It can serve to supplement the regular military as an irregular reserve, or it can oppose it, for example to resist a military coup.
For much of the history of England, the military was controlled by Parliament, which had access to the resources to maintain a standing army. At various times, The Crown and Parliament were in strong disagreement, but Parliament's economic ability to use the army was counterbalanced by the Crown's traditional ability to call out the militia. As long as the army's weapons were not radically more powerful than the militia's, this balance of power was effective.
The early Puritan colonists of America considered the militia an important social structure, necessary to defend their colonies from Native American attacks. All able bodied white males were expected to be members of the town militia.
In the American Revolutionary War, colonial militiamen or armed citizens agreed to turn out for service at a minute's notice. The term minutemen is used especially for the men who were enrolled (1774) for such service by the Massachusetts provincial congress. These were also known as the "valiant farmers" who fought against the British at Lexington and Concord.
The Framers of the United States, in keeping with this tradition, gave Congress the power to "provide for organizing, arming, and disciplining the Militia," as well as, and in distinction to, the power to raise an army and a navy. The Second Amendment to the Constitution of the United States may have been intended to formalize this balance between the "well-regulated" militia and organized military forces. Considerable controversy exists in the US over this amendment, however, and the ability of even a well-regulated militia to resist a modern army is debatable.
The United States Consolidated Statutes Title 10 (Armed forces), section 311 (Militia: Composition and Classes), paragraph (a) states "The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard."
The National Guard is the largest of the state militia forces in the United States. It is under both Federal and state control, and both the President of the United States and the state governor can call up the National Guard. Many National Guard units have recently served in Iraq. This can lead to problems for states that also face internal emergencies while the Guard is deployed overseas. Many of the states, such as New York and Maryland have organized state militia forces or State Guards which are under the control of the governor of a state and used to augment the National Guard.
There is also a political movement that calls itself 'militia' and is based on the common-law concept of an armed citizenry. These are not formally linked to a state or to the Federal government, and often oppose the Federal government because of what they consider oppressive policies. Most, but not all, are also opposed to illegal immigration, and several espouse white supremacy. The movement peaked in the early 1990s, but declined in popularity after the bombing of the Murrah Federal building in Oklahoma City, Oklahoma, on April 19, 1996.
One of the most famous and ancient militia are the Swiss militia. It is not widely recognized, but Switzerland is the most militaristic society on Earth, maintaining more than twice as many active-duty soldiers per capita as the next-most-militaristic country, Israel, and a trained, mobilizable reserve militia of 36% of the total population. However, it should be noted that Switzerland has a long tradition of political and military neutrality.
In Canada the word militia refers to the part-time army reserve component of the Canadian Armed Forces. Officers and soldiers in the militia train for one or two weekends a month and for two weeks a year. They can also volunteer for service with the regular forces including peacekeeping missions overseas.
Most Canadian cities have one or more militia units. Often these 'regiments' perpetuate famous Canadian regiments that are no longer required as part of the regular forces.
Some militia regiments:
U.S. and English
Swiss
Canada