I am sorry about the triple pings! Sure don’t know how that happened!
I have confidence that our US Military will NOT fire on their fellow Americans!
They've done it in the past, ie, the tent encampment in DC after WWI. To name one.
But even if we concede that troops wouldn't fire on civilians, here's what could happen.
Troops come around in a Martial Law or similar scenario. A hothead fires at the troops. They return fire and the furball begins.
It has happened with the Ohio National Guard at Kent State.
Kent state shootings -May 4 massacre of unarmed college students by the Ohio National Guard;
http://en.wikipedia.org/wiki/Kent_State_shootings
The students had been protesting against the American invasion of Cambodia. Guardsmen fired 67 rounds in 13 seconds, killing 4 and wounding 9 others.