>>South Carolina threatened to secede

As I remember, Massachusetts pad previously threatened - over what became the war of 1812. There were claims that it was about freedom of navigation, but I see it as more of a war against the Iroquois - and their allies-by-treaty, the British. Not that the Brits didn't want to take up hostilities against their upstart colony, smack their navy down. But on our side, it was about a land grab; folks on the interior couldn't give a stuff about maritime shipping. (Disclaimer, I'm from MA)

All that said, I'd 100% agree that dissolution (secession) was all about slavery, or more properly, the fear of it being outlawed. I see a lot in common between those southern 'fire breathers' who pushed secession and many members of the Trumpublican party today.

There had been animosity between colonies back in the colonial period (one of the reasons the Brits thought there's never be a successful revolt, probably why Howe was so 'kid gloves' at the start).

If anything, the Federal system came out of that mutual distrust. That animosity never fully went away.

As I remember (again, I'm too lazy to look it up) one of the causes of agita in the North was Southern states discarding pamphlets sent in the mail as the mail being carried was something expected. I think that regional attitude toward the Post Office continues even now.

That pre civil war period is spectacularly interesting - and might have lessons for us today. Thanks for bringing it back into focus.