The Posse Comitatus Act of 1878 prohibited the use of military forces for law enforcement duties within the boundaries of the United States in the absence of a declaration of martial law. British soldiers exercising such functions in the colonies was one of the proximate causes for the Revolutionary War. It has been a foundational principle ever since that our military is not to be used against its own citizens except in times of crisis. In the 1807 Insurrection Act, this was taken to mean “any insurrection, domestic violence, unlawful combination, or conspiracy.” In the 2007 Defense Authorization bill, a section was slipped in written by John Warner (R-VA) and Carl Levin (D-MI) which expanded the situations covered by the 1807 act to include “natural disaster, epidemic, or other serious public health emergency, terrorist attack or incident, or other condition.” It is this “other condition” which is the problem since this can mean anything and gives the President a blank check to use federal troops for any purpose within the borders of the United States. In the 2008 Authorization Defense bill, the language was refined to permit the use of federal troops in instances of threats to Constitutional rights. This is still vague and in any case Bush in a signing statement rejected even this limitation.
This is not a theoretical discussion. A September 8, 2008 article in the Army Times reports that beginning October 1 an active duty Army combat brigade will be assigned for use within the US. Among its missions are “civil unrest and crowd control.” It is described as the “first ever nonlethal package that the Army has fielded” and will be equipped with bean bag bullets, tasers, spike strips, shields, and batons. What is important to realize is that this is both a reaction to the gutting of the National Guard (item 48) and a means to suppress legitimate dissent in contravention of First Amendment guarantees.