http://www.strategypage.com/htmw/htpara/articles/20111101.aspxNovember 1, 2011: The purpose of State Defense Forces (SDF) in the United States is to have an organized and disciplined force to take up the peacetime roles of a state's National Guard especially in the responsibility of emergency response. The National Guard (originally the colonial militias) has gradually become a form of reserve force for the Department of Defense, and can be "federalized" (mobilized for service with the regular military) on short notice. This leaves states short of forces. For non-Americans, it's important to remember that the fifty states in the United States retain many powers and structures of sovereign states. It's not called the United "States" for nothing.
Meanwhile, the combat and military police roles of several SDF's have been eliminated or reduced. There are several reasons for this. One is tactical; there is a perceived need for emergency response forces, not back up for the police. The other is pragmatic; the high cost of insurance for an authorized armed force.
By statue or by simple preference state defense forces are open mostly to retired soldiers, sailors and airmen. Some state forces are holding units of specialists and professionals who are activated to be advisers to local and state authorities in times of emergency and have no tactical formations. In that instance prior military service is not an issue. Some SDF have aviation units with the aircraft provided by the members. They are trained in performing emergency support missions.
While these state forces cannot come under the command of Federal authorities, they have to be certified by the Office of Military Support in the National Guard Bureau so that they can purchase surplus military equipment. The SDFs are heavily involved in creating Civil Emergency Response Teams and they are a major tool for gaining recruits for state defense forces. Typically most state defense forces are authorized at manning levels that are cadre level. Most SDF's do not have any pay for its members and are purely voluntary.