Donuel said: He can't issue an order to have the Army to attack the demonstrators and or looters due to the Posse Comitas law. Unfortunately, not true. After the greatly muddled election of 1876, we still have no idea who really won, a compromise was made to put Hayes in office. In return for southern delegates Hayes withdrew all federal troops from Southern states, effectively ending Reconstruction. With southern politicians back in charge they tried to return as much as possible to the "good ol' days". Of course. the 13th amendment kept them from bringing back chattel slavery, but sharecropping scams, prison farms and vagrancy laws created new forms of forced labor that lasted well into the 20th century. As southern politicians returned to the U S Congress they pushed through Posse Comitatus to prevent military interference with these policies. This law did not keep Eisenhower from using the military to desegregate southern schools in the 1950s because one of several limitations to Posse Comitatus is Federal troops used in accordance to the Insurrection Act. Bottom line is that the Comitatus law is superseded by the Insurrection Act of 1807.
Most presidents have avoided this option for many reasons, not least of which is that the military is not trained in law enforcement and crowd control and is reluctant to square off against U S citizens. The last time troops were used in this regard were the Rodney King riots in L A in 1992 after Bush was asked for assistance by the governor. Using Armed Forces for policing without the consent of a state governor would have such a stench of autocracy that most presidents simply wouldn't consider it, but we all know that Trump doesn't care.