“What I don’t understand is your focus on ‘the national guard’. Wouldn’t the regular Army and Marine Corps be the ones to use?”
Wikipedia: The Posse Comitatus Act is a United States federal law passed on June 18, 1878, after the end of Reconstruction, with the intention of substantially limiting the powers of the federal government to use the military for law enforcement. The Act prohibits most members of the federal uniformed services (today the Army National Guard, Air Guard, and State National Guard forces when such are called into federal service) from exercising nominally state law enforcement, police, or peace officer powers that maintain “law and order” on non-federal property (states and their counties and municipal divisions) within the United States.
The statute generally prohibits federal military personnel and units of the National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress.
I think a rebellion, i.e., Secession, would see the Armed Forces quickly authorized by the Congress to be useable. At the present time, IMO, there would not be enough regular Army forces within U.S. borders to counter a Secession by multiple states. That could change after all the troops come home from Iraq; however, a conflict with Iran (which seems to be coming down the pike) would cause redeployment of the same forces.
To add to the problem of the U.S. military forces, the National Guard is currently highly mobilized to stations overseas, and so is also understrength as far as having to carry out operations within the U.S. boundaries.