The historical record fully support the conclusion that slavery was the casus belli. But that doesn't mean that people today who support secession or whom believe that Southern secession was justified share those same beliefs regarding slavery.
Time out. I am a man of the right, and I do not agree that the South was justified in secession on any level --- Constitutionally, morally, or ethically. Don't speak for me when you speak of secession as legitimate. It is not and never has been. I see it as a revolution without just cause.
But I also see it as having been inevitable --- destined to happen from the first shots at Lexington & Concord. The best efforts of the Founders, Framers of the Constitution and many brilliant and well intentioned men during the first eight decades of the Republic could only delay the inevitable clash.
And from a spiritual standpoint, I think Lincoln was right when he said that the War and the bloodshed was God's punishment on all Americans, whom he had blessed with so much, for the sin of slavery.