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.

A military reserve force is a military organization whose members (reservists) have military and civilian occupations. They are not normally kept under arms, and their main role is to be available when their military requires additional manpower. Reserve forces are generally considered part of a permanent standing body of armed forces, and allow a nation to reduce its peacetime military expenditures and maintain a force prepared for war. During peacetime, reservists typically serve part-time alongside a civilian job, although most reserve forces have a significant permanent full-time component as well. Reservists may be deployed for weeks or months-long missions during peacetime to support specific operations. During wartime, reservists may be kept in service for months or years at a time, although typically not for as long as active duty soldiers.