For an example of what happens when an army is used against its own population, Mexico 40 years ago provides an example. True that Mexico had foreign agents manipulating and penetrating its national political scene at the time of this massacre in Mexico City, but the US has some of the very same subversion today if not to the same extent. Certainly today’s Israeli agents infiltrating into the US military political structure are not totally akin to US agents infiltrating into the Mexican power elite back in 1968 as continues to this very day.

Tlatelolco – the 40th anniversary
Mexico, October 2, 1968: The Night of Tlatelolco; the Death of the Student Movement

Still, many armies have been used against their own populations and there is nothing unique or different about what American troops would do at home than from what Mexican troops did in Tlatelolco, if American troops were called upon by their officers to fire on American citizens. The nature of the military anywhere is to follow orders without any real questions of the leadership. That usually only will happen only if the military in question is disintegrating in total rout. Meanwhile….. they will shoot if called upon to do so.