They're already supposed to be, to an extent. That's why they're called "states".
If the South was to secede again, would the federal government have the stomach to wage war like it did between 1861 to 1865?
Sure. They always have the stomach when the opponent is a good person who just wants to be left alone.
Certainly you are not asserting that the slave power, that wanted continued authority to rape, torture, and kidnap, were good people motivated by good intentions?
That the people who demanded that fugitive slaves in northern states be returned with the cooperation of Northern state official wanted to be left alone?
Gracious! What whoppers!