It all depends on what you think the role of President should be: should it be to preserve the boundaries of the country at all costs or to respect the wishes of one portion of the country that wishes to dissolve the bonds that have tied it to another? Slavery was the issue that led to secession, as is clear from reading the Texas secession ordinance or any other southern diatribes of the time.
But secession did not automatically mean war. Buchanan decided he did not have the constitutional authority to make war to force the south to stay in the disunited states. He thought the war would be costly in extreme. He was right as to the latter at least.
As to whether he was correct in thinking that it was the south's right to leave, I would make the following suggestion: No country lasts forever. Rome didn't, the greatness that was Athens did not, etc. All countries or empires or what have you have end dates. It is just a matter of when and what causes it. The US will not last forever. I don't know when it will dissolve or be dissolved but it is as certain as the sun making an appearance tomorrow morning.
The north and south had reached a point where they were not on the same page any more and both were fed up with each other. The south was stunned when John Brown was celebrated as a hero by the north. Stunned. He had tried to raise a slave revolt which would have resulted in the deaths of thousands of people who had no slaves as well as those who did. Read up on Nat Turner or Haiti and you will get an idea of how the south looked on that possibility. And their countrymen to the north were celebrating Brown's effort and turned him into a pop hero. Sort of like Che Guevara. The southerners decided they should not wait around for the next episode, especially considering that the north was getting to the point of demographic certainty to politically overwhelm them. Get out while the getting is possible.
Lincoln, in his certainty of his own convictions, thought he could force them back in easily. He asked for troops in a number under 100,000 for six months. He completely misread the depths of the resistance and the willingness to fight by the southerners, who believed his election was an existential threat---and they were right. I am not defending their determination to defend their labor system, merely pointing out that they thought it necessary.
The war came because the man who was elected president was an old Whig who worshiped Henry Clay and believed the Union was a mystical body that could not be rendered. So he waged war to force the southerners to stay against their wishes and half a million people got killed
When the war ended and the states were allowed to conduct elections the southerners elected the people who they thought best represented their interests. Just like we do now. I don't care for Abbott or Patrick or Paxton but I acknowledge that they were the choices of the electorate--and for the reasons why I don't like them. They won, my guys did not. So the obvious solution to that problem is to dissolve the state government and install a new one more to my liking, right?
A majority of southerners were unreconstructed. They thought they lost the war and had to be part of the US again; they did not think they lost the argument that led to the war. I think they made a mistake and should have embraced the idea of educating the freedmen and helped them find ways to sustain themselves----forty acres and a mule or a Kardashian or whatever it would take. This was not the path the southerners chose,