Even after serving 7 years of active duty, I am not sure the US military would not fire on civilians. IMHO.
Kent State was raw National Guard troops, and The Bonus Marchers weren’t fired upon (they were tear gassed and forced away at bayonet point).
The one thing that keeps me from believing our own soldiers would blindly obey orders to subjigate their own people is the fact that they treat foreigners well (how many threads on freerepublic were devoted to pictures of our boys making friends with the Iraqis and Afghans?), so I find it hard to believe they wouldn’t do the same with their own people.
Also, keep in mind that even Nazi Germany and the Soviet Union had to create special paramilitary units to police the civilian population, as the regular military disdained such duties.