American troops have been responsible for a number of massacres of American civilians in the past. Kent State is the most famous recent event.

It's not like they'd issue orders saying "go massacre all those people." For certain units, it's be easy. Phone coordinates and fire orders to an artillery company, for example. Even infantry can be easily manipulated: such and such a place has "domestic terrorists." Go in and seize their weapons. If they comply, mission accomplished. If they don't, and become violent, the soldiers will defend themselves.

A large part of the problems the US has been having with domestic police forces in recent years can be traced back to the preferential hiring of vets. They're already trained to treat civilian populations as hostile, and it's clear that that training has transferred to apply to the American civilian population.