"The Army National Guard's charter is the Constitution of the United States. Article I, Section 8 of the U.S. Constitution contains a series of "militia clauses," vesting distinct authority and responsibilities in the federal government and the state governments. These clauses and follow-on legislation have sculpted the Guard as you see it today. Here are summaries that will help you understand how the Guard came to be what it is today."
Army National Guard: http://www.arng.army.mil