“From an American Constitutional point of view, the Confederate declarations of secession from the United States were unconstitutional.”
Show me where in the U.S. Constitution of 1861 that secession was unconstitutional, or illegal.
The Founding Fathers and early US Presidents believed that and took it for granted. Senator Stephen Douglas in 1860: “I am in favor of executing in good faith every clause and provision of the Constitution and protecting every right under it - and then hanging every man who takes up arms against it!”