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The Militia Act of 1903 divided what had been the militia into what it termed the “organized” militia, created from portions of the former state guards to become state National Guard units, and the “unorganized” militia consisting of all males from ages 17 to 45, with the exception of certain officials and others, which is codified in 10 U.S.C. § 311. Some states, such as Texas and California, created separate state defense forces for assistance in local emergencies. Congress later established a system of “dual enlistment” for the National Guard, so that anyone who enlisted in the National Guard also enlisted in the U.S. Army. – http://en.wikipedia.org/wiki/Militia