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.
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.