It's illegal according to the Posse Comitatus Act (http://en.wikipedia.org/wiki/Posse_Comitatus_Act), unless of course they want to claim they are using the troops pursuant to the Insurrection Act (http://en.wikipedia.org/wiki/Insurrection_Act) against terrorists or they're being used to stop the transport of nuclear material. That's the only time the military can be used against American citizens on U.S. soil.