An armed force of citizen soldiers. Originally, militia systems were based on the idea that every citizen was obliged to serve his country; George Washington's army consisted of 41 percent militia. The other justification for a militia is that it safeguards the country against the possibility of gross abuse of power by a government or professional army. The Second Amendment of the U.S. constitution states, "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear arms shall not be infringed," a clause that is hotly debated today by gun control advocates and their opponents.

The state militia were replaced in 1916 by the National Guard. However, during the 1990s there was a resurgence of interest in the idea of a citizen's militia, and many states now have such organizations. Some of them are dominated by right-wing patriots and believers in conspiracy theories, who believe the U.S. government is becoming a tyranny and they must take steps to defend themselves against it before it is too late.