State Defense Forces

State defense forces (SDF; also known as state guards, state military reserves, or state militias) in the United States are military units that operate under the sole authority of a state government; they are partially regulated by the National Guard Bureau but they are not a part of the Army National Guard of the United States. State defense forces are authorized by state and federal law and are under the command of the governor of each state.

State defense forces are distinct from their state's National Guard in that they cannot become federal entities. All state National Guard personnel (to include the National Guard of the District of Columbia, the Commonwealth of Puerto Rico and the territories of Guam and the Virgin Islands) can be federalized under the National Defense Act of 1933 with the creation of the National Guard of the United States. This provides the basis for integrating units and personnel of the Army National Guard into the U.S. Army and, since 1947, units and personnel of the Air National Guard into the U.S. Air Force.

The federal government recognizes state defense forces, as per the Compact Clause of the U.S. Constitution, under 32 U.S.C. § 109 which provides that state defense forces as a whole may not be called, ordered, or drafted into the armed forces of the United States, thus preserving their separation from the National Guard. However, under the same law, individual members serving in the state defense force are not exempt from service in the armed forces (i.e., they are not excluded from the draft). Under 32 USC § 109(e), "A person may not become a member of a defense force ... if he is a member of a reserve component of the armed forces."

Nearly every state has laws authorizing state defense forces, and 22 states, plus the Commonwealth of Puerto Rico, have active SDFs with different levels of activity, support, and strength. State defense forces generally operate with emergency management and homeland security missions. Most SDFs are organized as army units, but air and naval units also exist. As governors often use state defense forces to augment their state's Army National Guard and Air National Guard units, state defense forces have been both officially and informally called National Guard Reserves.

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