>The takeaway, at least for me, is that both men were not that far apart on the topic of slavery, even though that's the primary lens through which Americans now view the conflict.
Hindsight is 20/20: the conflict was primarily about slavery. The Confederates said so themselves, and they even wrote it explicitly into their constitution. The northern states were pushing to ban slavery, and were refusing to honor the southern states' laws regarding escaped slaves, and the south could see that they were going to be forced to abolish slavery before long. So they rebelled. They've tried ever since to frame it as the more vague "states' rights" issue, but the real issue was slavery; it's the whole reason they wanted to secede.
>Which leaves us with this question: Is membership in the Union voluntary, or compulsory? I'm not sure the answer provided by the civil war is morally correct.
You make a good point here, one that I've wondered about for ages. I agree, political units should be able to voluntarily leave unions if they want, due to the principle of self-determination. Should Ukraine be forced back into a union with Russia? I don't think so. Similarly, if Kaliningrad wanted to secede and form a new country and then join the EU, they should have that right. So why shouldn't the southern US states be allowed to secede? At the time, the slavery issue dominated, as it was an obvious human rights issue. But suppose Florida in 2025 wants to secede and become a theocratic apartheid state with slavery legalized; should that be allowed? It seems like many would say no, because this would lead to human rights abuses and by keeping them in the union, that can be avoided. Or suppose California, Oregon, and Washington decide to all secede because they don't want to be in a union with the crazies in Florida and are sick of having the world's 5th-6th largest economy but not having universal healthcare or high-speed rail because the other states are dragging them down and taking their tax money; should they be allowed to leave and form a new country? In practice, it seems that political units only succeed in seceding when 1) they have a violent conflict and win (e.g. American revolution), or 2) the larger entity either agrees (e.g. Czechoslovakia, Brexit) or just doesn't have the power or political will to stop it (e.g. USSR, British Empire).