"Relatively easy"
not like the entire economy was destroyed and entire cities burned to the ground.
And I'd like to point out that the official casus belli for the Civil War was the shelling of Fort Sumter, not secession.
The occupation didn't go as planned because of Lincoln's VP taking over. The intent was go a few years with the south under military rule and forcefully integrate the freed slaves. Instead southern generals and soldiers were given amnesty almost immediately, the Southern states were re-added to congress with no fuss, and southern cops were given autonomy. Hence sharecropping, vote suppression and the KKK.
That interpretation treats the confederacy as a separate nation rather than rebels in a civil war. The north didn't have or need a reason to start the war because they didn't start the war; the southern rebels did by being rebels.
I love the double think of a country built on states seceding from another country forbidding states from seceding from their own. "God gave us the right to rebel, but not the right to rebel against us!"
I mean it would be, except that the founding fathers knew they were getting into a war, knew they were breaking English law, and they knew what would happen if they lost. Their argument wasn't "we have an innate right to do this" it was "fuck King George in particular." The stuff about having a right to take up arms was explicitly against tyranny. By their own rhetoric, if the king had been just and given them a voice in parliament, they would have owed him loyalty as English citizens. Now the confederates believed they were up against tyranny, don't get me wrong. But the founding fathers would never have been stupid enough to call their rebellion "the war of English aggression." And the north is hardly going to call their own government tyranny, especially since Lincoln hadn't really done all that much at the time of the rebellion.
To be clear, while I have a problem with the doctrine of secession, that's not my
big level problem with the Confederacy. If someone in USA history had started a rebellion for a good reason, even using a dumbass justification like secession, I would at least consider the idea that they were in the right.
*although you could argue they cared more about their own wallets then freedom for the masses, but that's a whole other digression and not one I have a strong opinion on