Eugene Volkh conducts a legal analysis of the assertion that Appomatox Courthouse permanently settled the issue of secession.
[A judgment on secession] must be a judgment based on how we see the world today, not based on what happened 144 years ago. A matter is “settled” by political decision only so long as the political decision commands the adherence of the polity. If in 2065 Alaska, California, Hawaii, or Texas (just to consider some examples) assert a right to secede, the argument that “in 1865, the victorious Union government concluded that no state has a right to secede in opposition to the wishes of the Union, so therefore you lack such a right” will have precisely the weight that the Americans of 2065 will choose to give it — which should be very little.
And beyond that, even if there is some precedent of some sort properly set by the Civil War (and I continue to disagree that there is), any such precedent can’t tell us much about consensual secession. The talk I occasionally hear of secession (again, talk that I think is not really serious) is not about departure in the face of military opposition — it’s about creating a political sentiment in some place in favor of seceding, and a political sentiment in the rest of the country in favor of allowing the secession. The results of a bloody civil war tell us nothing about the propriety of a Velvet Divorce.
It’s an interesting point. Just because something was held to be illegal in the past does not mean that it is illegal now. And the fact that secession was deemed illegal by political fiat rather than a constitutional or legislative means lends strength to this argument. There’s also the very simple fact that what was once seen as a just means to go to war in the past may no longer be seen as a just means for another war. The issue of secession was also so closely ties to slavery that it is difficult to predict how it would be approached without such a major issue overshadowing it.