I've got a different take on this than the OP:

I spent a couple years stationed at Marine Barracks 8th and I in Washington, DC. The unit there is organized as a "short" Infantry Battalion and is tasked with many duties, mostly ceremonial in nature.

One of the non-ceremonial duties is to be organized, trained, and equipped to support law enforcement in a martial law/riot/disaster scenario.

We liked the training because it's fun to beat the shit out of each other, but a lot of guys really thought about what it would mean to do it for real. Some of the guys who were more interested in politics (including me) didn't like it much.

During one of the bull sessions we had, I asked one of my Marines if he would obey my direct order to fire live rounds into a rioting crowd on The Mall. His response (paraphrased after many years, but accurate):

"You know I respect you, Sergeant Henry, and I'll do whatever the fuck you say needs to be done. But I don't think I could do that. That's a cop thing, and I'm a fucking Marine, not a cop."

Now, this was a 19 or 20 year old enlisted Marine rifleman, in a very disciplined unit. This is a guy who would jump on a grenade for his buddies, and happily kill all left-handed redheads in some third world shithole if I told him to. He said (and I believe him) that doing that kind of stuff to other Americans was wrong and not something he would do.

I told him that was the right answer, by the way.

Now, I know for a fact that if some Occupy D.C. type people lost their goddamn minds and started, say, throwing Molotov cocktails at the Commandant's House they'd die a quick, messy death.

I am less sure about the same response in regards to the White House or the Capitol Building, but I could visualize some bad shit going down, short term.

As far as peaceful demonstrations go - I highly, highly doubt the US military being willing to use lethal force. We learned a lot from Kent State (not federal troops, by the way) and the Bonus Marches.

Fuck with the Commandant, though, and you're gonna die.

Also, keep in mind that the US military does not have the same ethos as, say, Libyan forces or even the Czechoslovakian military of the 1980's. Most of the military is very much a Citizen's Military. There isn't a very large professional warrior class - most folks do a few years and get out. We swear oaths to The Constitution, not to any political party or individual. Hell, go read about the last time somebody powerful tried to co-opt a high ranking service member (Gen Smedley Butler) and what happened.

One of the mainstays of morale in the US military is the belief that we do horrible shit so Joe and Jane Civilian don't have to. You start asking the military to do horrible shit to Joe and Jane, and one of the pillars of "Why We Fight" is gone. All it takes is a few guys like the young Marine in my example above, and unit cohesion, combat power, and morale would be absolutely shot.

My best guess is that using the US military to suppress nationwide riots a la Syria would cause the military to collapse in less than a week. You could build a US military that would do that kind of work, but you'd need decades. You'd need to purge/retire the present Officer and NCO corps, and totally re-vamp the training/indoctrination system.

I'm a fucking die-hard former Marine. I'd still be in if I hadn't gotten blown up, probably. If I met that douchebag "AllCombatPersonel" out in a bar and we were both in civilian clothes, I'd happily smack the shit out of him.

If I still wore the uniform, though - no fucking way. Even though he doesn't understand it, doesn't want to believe it, and doesn't deserve it - I worked for him. I'd destroy a lot of shit and willingly die so that not a pwecious wittle hair on his empty, worthless head was harmed.