The State Defense Force, also known as the State Guard or the State Military Reserve, is an organized, all-volunteer military organization, which serves primarily within its own respective state and communities.  

The SDF is not part of the National Guard or the Federal military force, though it may work in association with or in support of them. It generally works in support of local communities or governments near where SDF units are based, in Emergency Management, community support, support to local events, or organizations such as hospitals or sheriff's departments.  Many SDFs are provided for in their respective states' statutes.  For example:  the South Carolina State Guard (SCSG),http://www.scsg.org , is one of the oldest SDF organizations, provided for in Title 25, Chapter 3, SC Code of Laws; New York and Texas are two old and well-established SDF organizations:(see http://www.dmna.state.ny.us/nyg/nyg.html and http://www.txsg.state.tx.us, respectively); the Maryland Defense Force (MDDF) was established under Article 65 (Militia Law) in the Annotated Code of Maryland.

All active SDF organizations recognized by their states are subject to the authority of their state's Adjutant General and Governor. They fulfill missions approved by these officials and may at the discretion of the Governor augment the forces of their state's National Guard.  The SDF and its members are not subject to call by the federal government, and do not leave the state without permission of the Governor.  On occasion a SDF unit may travel across its border where its state has mutual support agreements with adjacent states, or where the Governors permit a SDF unit to travel across their respective borders.  For example, after 9/11 a South Carolina State Guard unit was permitted to travel to New York City (NYC) for a month to provide security at the St. Paul 's Chapel shelter adjacent to “Ground Zero” and the New Jersey State Guard provided support and protection for NYC’s western waterways.