Not just that. Just a few years back, Czkeckoslovakia split into the Czech Republic and the Slovak Republic without any bloodshed in a nice civilized manner. A few years before that the former republics of the former USSR, split into their individual 15 independent republics like Georgia, Azerbaijan , Turkmenistan etc, without that much trouble. So it can be done.
” Nonetheless, violent secession can lead to great friendships. England is probably our greatest ally.”
Been known to happen.
Difference, though, is that those countries were not great military powers, much less superpowers, much less the world's only superpower.
I shudder to think of a world in which the United States is unable to project its superior military power.
That said, if the split was a "virtual" one -- where, because of a return to federalism, states were allowed to reinvent themselves in their own image -- the union would be preserved and the one great task of the federal government would be to focus on national security and military defense.