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The reserve components of the United States Armed Forces are military organizations whose members generally perform a minimum of 39 days of military duty per year and who augment the active duty (or full-time) military when necessary. The reserve components are also referred to collectively as the Guard and Reserves.
According to 10 U.S.C. § 10102, the purpose of each reserve component is to provide trained units and qualified persons available for active duty in the armed forces, in time of war or national emergency, and at such other times as the national security may require, to fill the needs of the armed forces whenever, during and after the period needed to procure and train additional units and qualified persons to achieve the planned mobilization, more units and persons are needed than are in the regular components.
The eight reserve components of the U.S. military are:
Note: The Army National Guard of the United States and the Air National Guard of the United States primarily fall under the National Guard of the United States.
The civilian auxiliaries of the U.S. military are not considered to be reserve components of the respective services but could assist the military in peacetime or wartime; the exception is the Coast Guard where upon determination by the Commandant Auxiliary members become part of the temporary Reserve (per the CG Authorization Act of 1996):
During times of war, the Merchant Marine is classified as part of the uniformed services and members obtain veteran status.
Additionally, the state organized militia such as various naval militias and state guards (a.k.a. state defense force) are not considered reserve components because they are not federally recognized even though they may perform a military function. However, members of the National Guard, which is a state's primary militia force, can be mobilized to support federal requirements becoming part of the National Guard of the United States.
The reserve components are the embodiment of the American tradition of the citizen-soldier dating back to before the American Revolutionary War. They are regionally based and recruited (unlike their active duty counterparts) and, in the case of the Army and Air National Guard, are the organized state militias referred to in the U.S. Constitution. Members of the reserve components are generally required to perform, at a minimum, 39 days of military service per year. This includes monthly drill weekends and fifteen days of annual training (giving rise to the old slogan “one weekend a month, two weeks a year”).
While organized, trained, and equipped nearly the same as the active duty, the reserve components often have unique characteristics. This is especially true of the National Guard, which performs both federal and state missions. In addition, reserve components often operate under special laws, regulations, and policies.
The Reserve Components of the United States Armed forces are named within Title 10 of the United States Code and include: (1) the Army National Guard of the United States, (2) the Army Reserve, (3) the Navy Reserve, (4) the Marine Corps Reserve, (5) the Air National Guard of the United States, (6) the Air Force Reserve, and (7) the Coast Guard Reserve. In practice the use of the term “reserve” varies depending on the context in which it is used. In one context, as used here in this article, it applies to all seven of the reserve components of the U.S. military. In another context, it applies to only the five reserve components directly associated with the five active duty military services but neither to the Army National Guard nor the Air National Guard.
In most respects, the Army National Guard and Air National Guard are very similar to the Army Reserve and Air Force Reserve, respectively. The primary difference lies in the level of government to which they are subordinated. The Army Reserve and Air Force Reserve are subordinated to the federal government while the National Guards are subordinated to the various state governments, except when called into federal service by the President of the United States or as provided for by law. For example, the California Army National Guard and California Air National Guard are subordinated to the state of California and report to the governor of California as their commander-in-chief.
This unique relationship descends from the colonial and state militias that served as a balance against a standing federal army, which many Americans feared would threaten states’ rights. The militias were organized into the present National Guard system with the Militia Act of 1903.
Besides the theoretical check on federal power, the distinction between the federal military reserves and the National Guard permits state governors to use their personnel to assist in disaster relief and to preserve law and order in times of crisis. The latter is permitted because the National Guard are not subject to the restrictions of the Posse Comitatus Act unless they are under federal jurisdiction. The restrictions, however, do apply to the four of the other five reserve components just as it does with their active duty military counterparts. The United States Coast Guard and United States Coast Guard Reserve are not subject to the restrictions of the Posse Comitatus Act because they are the only Armed Force of the United States that is not part of the United States Department of Defense.
While the National Guard is a militia force organized by each state, the National Guard of the United States is a reserve federal military force of the United States Armed Forces. The National Guard of the United States is joint reserve component of the United States Army and the United States Air Force and are made up of National Guard members from the states appointed to federal military service under the consent of their respective state governors. The National Guard of the United States maintains two subcomponents: the Army National Guard of the United States for the Army and the Air Force's Air National Guard of the United States. The Army National Guard of the United States is made up of federally recognized members of the Army National Guard  and the Air National Guard of the United States is made up of federally recognized members of the Air National Guard.
All members of a reserve component are assigned to one of three reserve component categories:
Individual service members or entire units of the reserve components may be called into active duty (also referred to as mobilized, activated, or called up), under several conditions: