It really depends on how vicious whatever commanding force controls the army would want to be.
If they're willing to rule over a country of bones, the asymmetric warfare resources the United States has available are sufficient to put down any rebellion. How much fight do you think 81 million armed civilians would keep in them if cities just started vaporizing?
A hypothetical American government with open rebellion wouldn't need to take and hold territory... It could decide specific chunks of the country are worth saving and nuke vast quantities of the rest.
I'd say that scenario is impossible, but this is America we're talking about... If there's one thing the history of American warfare teaches, It's that you can't apply the rest of the history of war to the way America fights wars. This is the country that gave us, in modern times, ending a rebellion by putting civilian cities to the flame until the civilians became too tired and hungry to support the war.