In a classic article in the Journal of American History, which was based on his presidential address to the Organization of American Historians in 1978, the great Civil War historian Kenneth Stampp made the claim that the arguments in favor of the constitutionality of secession made by the Southern states were as strong, if not stronger than the constitutional arguments made, then and now, in opposition to secession. Stampp is to my mind the greatest Civil War historian of the 20th century and his views on secession remain required reading and are cited routinely today. This is not to say Stampp was correct, only to use his classic article on the 150th anniversary of secession as a jumping off point for reconsidering the legality and constitutionality of secession and also, I think, to go to first principles to consider whether it is possible or useful to definitively try and answer the question: was secession legal?
Hamilton, Daniel W.
"Still Too Close to Call? Rethinking Stampp's "The Concept of a Perpetual Union","
Akron Law Review: Vol. 45
, Article 2.
Available at: https://ideaexchange.uakron.edu/akronlawreview/vol45/iss2/2