During colonial America, all able-bodied men of certain ages were members of the militia, depending on the respective states rule. Individual towns formed local independent militias for their own defense. The year before the US Constitution was ratified, The Federalist Papers detailed the founders' paramount vision of the militia in 1787. The new Constitution empowered Congress to "organize, arm, and discipline" this national military force, leaving significant control in the hands of each state government.
Today, as defined by the Militia Act of 1903, the term "militia" is used to describe two classes within the United States:
Congress has organized the National Guard under its power to "raise and support armies" and not its power to "Provide for organizing, arming and disciplining the Militia". This Congress chose to do in the interests of organizing reserve military units which were not limited in deployment by the strictures of our power over the constitutional militia, which can be called forth only "to execute the laws of the Union, suppress insurrections and repel invasions." The modern National Guard was specifically intended to avoid status as the constitutional militia, a distinction recognized by 10 U.S.C. Sec 311(a).
The term "militia" derives from Old English milite meaning soldiers (plural), militisc meaning military and also classical Latin milit-, miles meaning soldier.
The Modern English term militia dates to the year 1590, with the original meaning now obsolete: "the body of soldiers in the service of a sovereign or a state". Subsequently, since approximately 1665, militia has taken the meaning "a military force raised from the civilian population of a country or region, especially to supplement a regular army in an emergency, frequently as distinguished from mercenaries or professional soldiers". The U.S. Supreme Court adopted the following definition for "active militia" from an Illinois Supreme Court case of 1879: " 'a body of citizens trained to military duty, who may be called out in certain cases, but may not be kept on service like standing armies, in times of peace'. . . when not engaged at stated periods . . . they return to their usual avocations . . . and are subject to call when public exigencies demand it."
The spelling of millitia is often observed in written and printed materials from the 17th century through the 19th century.