They rebelled and they lost.
If those in the armed forces will refuse to kill, wound, and destroy their fellow countrymen, there is a real chance that the secession will succeed.
What if they don't refuse?
As far as the Armed Forces goes, I do not know what they will do, nor does anyone else. I suspect that they will do whatever they are ordered to do. If the government decides to confiscate our guns, the military will enforce that, too.
Every law passed by Congress is backed up by force, and that means military force if needed. If a state is forced to submit to a centralized government, how is that different than fascism or communism? Totalitarianism in any form is still totalitarianism. Whatever happened to the men that once said, “Give me liberty or give me death”?