The Bonus Army ordeal is just one of many instances the military was used against US citizens.
#2 If SHTF and the Army doesn't turn on the American people the these so-called elected thugs in DC would flee. They would have no power.
#3 The Kent State shootings were by a national guard unit shooting at pro-socialist/NVA students.
#4 the Posse Comitatus Act prohibits the military from being used inside the country for law enforcement domestic functions.