The National Guard is a state-based military force that becomes part of the U.S. military's reserve components of the U.S. Army and the U.S. Air Force when activated for federal missions. It is a military reserve force composed of National Guard military members or units of each state and the territories of Guam, the Virgin Islands, Puerto Rico, and the District of Columbia, for a total of 54 separate organizations. It is officially created under Congress's Article 1 Section 8 ability to "raise and support armies". All members of the National Guard are also members of the organized militia of the United States as defined by 10 U.S.C. § 246. National Guard units are under the dual control of state governments and the federal government.

The Army National Guard (ARNG), in conjunction with the Air National Guard, is an organized militia force and a federal military reserve force of the United States Army. They are simultaneously part of two different organizations: the ARNG of each state, most territories, and the District of Columbia, as well as the federal ARNG, as part of the National Guard as a whole. It is divided into subordinate units stationed in each state or insular area, responsible to their respective governors or other head-of-government.