I think when (not if) the country comes apart it won't be a 21st century replay of the Civil War. More likely, it will be the withdrawal by right leaning states from polite discourse with the federal government.

We've already seen this on both sides of the political spectrum with the left refusing to enforce immigration law and the right beginning to refuse to enforce federal gun laws. People don't realize that everything from the momentous to the mundane relies on the willing cooperation of the states with the federal government.

Did you know that the states actually have a formal agreement with the federal government to tell them when people are born and die? There is no federal database of this information, so they have no idea to turn off the Social Security benefits of a dead person unless the state they die in shares that information.

This is already been litigated many times in the Supreme court. There is no federal authority to compel states to enforce federal law. Simply refusing to play along will cripple the federal government and leave them with little recourse. They can cut off the flow of federal dollars, but hopefully the states have established a degree of financial Independence before opting out of playing with the feds.

The only other choice would be an armed intervention, which violates the constitution, and provides the casus belli for the states to actually fight back. Forcing the feds to fire first strips them of their moral superiority and brings more people over to the cause.