structural:scholar_edition, information:classification
20th century

The Militia Act of 1903 divided what had been the militia into what it termed the "organized" militia, created from portions of the former state guards to become state National Guard units, and the "unorganized" militia consisting of all males from ages 17 to 45, with the exception of certain officials and others, which is codified in 10 U.S.C. § 311. Some states, such as Texas, California, and Ohio, created separate state defense forces for assistance in local emergencies. Congress later established a system of "dual enlistment" for the National Guard, so that anyone who enlisted in the National Guard also enlisted in the U.S. Army. When the U.S. Air Force was established as an independent service in 1947, the National Guard was further divided into the Army National Guard and the Air National Guard. Under this construct, the 1933 defense act's "dual enlistment" facet was further amended so that enlisted soldiers and commissioned officers in the Army National Guard were also enlisted or commissioned in the Reserve Component of the U.S. Army. Enlisted airmen and commissioned officers in the Air National Guard were also enlisted or commissioned in the Air Reserve Component (ARC) of the U.S. Air Force.

The 20th century saw the rise of militia organizations in the United States, these private militias often have an anti-government outlook and are not under the civil authority of the states. Privately organized citizen militia-related groups blossomed in the mid-1990s. Many militia groups are based on religious or political extremism and some are regarded as hate groups.