Americans no longer have the stomach to murder millions of their own countrymen over a matter of political realignment.
If one or more states decided to walk away from the union, there'd be a whole lot of racket and maybe even some implied threats from Washington, but it would stop short of actually sending troops in to quell an alleged 'rebellion'.
This is a far different age with a very different people than what existed in the mid nineteenth century. Recent administrations have a hard time getting support from the American people to conduct military operations against known enemies, let alone against their own people over a purely political matter.
You are flat out wrong on that. The people in control of this country would never allow it. If it does come down to secession, it will be a bloodbath. Personally, I think secession won’t happen and instead the country will one day descend into civil war. It’s not so much red state vs. blue state but city/ suburb vs. small town/ rural these days.