I don't think you understand the sheer orders of magnitude more capable of death and destruction the government of the USA is compared to "We the People". The threat of repercussion doesn't exist when you can carpet bomb or nuke anybody who decides to oppose you with an order or a push of a button. In this day and age it's entirely possible to murder individuals with orbital satellites and drones from half the planet away. If the American people decided to rebel against a unified government that wanted to oppress them, the rebels would never even see a government soldier until the rebels were already so beaten down by the bombings they'd rebels would be begging for the government to stop. It would go something like this:
US Government: Hi! We've decide we're going to burn the entire US Constitution on live television! There are no rights and you exist only at our whims and die at our whims.
Texas: Oh yeah? Come on guys! Now we justify all those guns we bought. We're rebelling!
US Government: Oh, okay. Pushes button and Houston is obliterated.
Texas: PLEASE STOP! We'll stop rebelling and do whatever you say!
US Government: Should've done that in the first place.
Florida: Those pathetic Texans might have given up, but we won't! We're rebelling.
US Government: Oh, okay Pushes button and Miami is obliterated.
Florida: PLEASE STOP! We'll stop rebelling and do whatever you say!
US Government: Anybody else? Hovers finger over button.
Other 47 states: Uh... no we're good.
Tennessee: Actually I...
Other 47 states: SHUT UP AND BOW TO OUR LORD AND MASTER!!!
Tennessee: Okay...
A military doesn't need boots on the ground to control the populace when they have such overwhelming power they can just make a few shows of force and then not only will the populace be controlled, the populace will control any dissenters that do appear for the military because of the fear of repercussions.
What would stop the government and the military is the rest of the government and military, and if whoever it was that decided to actually declare themselves absolute dictator chances are they would already have the government and military in their pocket anyway. Otherwise the senators, generals, mayors, etc. who want to keep their power would defy them and stop them. Let's be frank here, the only reason that this hasn't happened already is that everybody in the government and military is constantly working to keep and increase their own power at the expense of the rest of the government and military. The only thing holding this country together is that everybody is so selfish and power hungry they can't possibly work together to oppress the people... that much.