Slavery is the simplistic answer. But we have to remember that loyalty to states, and states’ rights, was greater than loyalty to the nation in our early history. The US tried a confederate form of government before our Constitution was ratified. There was a legitimate legal issue as to the right of states to secede, at least until the South fired on Ft. Sumter.
‘But we have to remember that loyalty to states, and states’ rights, was greater than loyalty to the nation...’
sure, sure, right up to 1850 and the Fugitive Slave Act, when it became ‘loyal’ for the state rights bunch to petition the federal government to return their precious escaped slaves...
Which they only did because Lincoln ordered an attack against them first.
Bet you didn't know that.