Fortunately, the US has strong institutions and balance of powers that make civil unrest, etc a rare event. When there is, actions depend on the situation.
Note that I'm not an advocate of arming one's self against the government in any way. I'm pointing out that the fantasyland of some citizen militia fighting any modern nation-state is absurd.
Examples of things where unrest or rebellion were put down by force:
https://en.wikipedia.org/wiki/Kent_State_shootings
https://en.wikipedia.org/wiki/Whiskey_Rebellion
https://en.wikipedia.org/wiki/Nat_Turner%27s_slave_rebellion
That's not to say these fears are completely unfounded -- they've been realized before many times throughout history. But when faced with questions such as, "Would the US military turn on the people?" they're likely to assess the risk far higher than the average person would. They're also far more likely to be libertarians.
We had a hard time getting soldiers to kill civilians in Vietnam. We had a hard time shooting others in WWII. How can you convince me that the majority of these soldiers are going to shoot their friends and family? That scenario itself seems ludicrous.
I'm not sure why the US military would be an exception in the history of hundreds or thousands of years of militaries being used against domestic civilian populations. Politicians are very good at getting militaries to heel. That's why militaries exist in the first place.
"We had a hard time getting soldiers to kill civilians in Vietnam. We had a hard time shooting others in WWII. How can you convince me that the majority of these soldiers are going to shoot their friends and family? That scenario itself seems ludicrous."
Some soldiers may have had a hard time killing Vietnamese civilians, yet despite that nearly 200,000 died. In WW2 tens of millions died. As for shooting friends and family, see the viciousness and bloodiness of the American Civil War.
Remember Kent State?
In the hypothetical scenario where there is armed insurgency against the US government and US military, I don't doubt that the US military would do what it had to do to suppress such an insurgency. Yes, there could be some dissent in the military against doing that, but short of an all-out civil war, they would obey.
> I, _____, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic