States and localities are required to follow federal law by the Supremacy Clause. It was not universally assumed that secession was constitutional.
I’d say that secession was a subject for consideration by the courts, but Jefferson Davis thought it was a war-worthy point.
What is said about the word ‘assume’ ?
Well three states said so explicitly at the beginning, and two of them were among the most important states in the nation. At least two more said so at the Hartford Convention in 1814, (Massachusetts and Connecticut) and so whether or not it was "universally assumed" can be proven, it can certainly be proven that significant numbers of important states believed it to be true.
The evidence in favor of legal secession far outweighs the evidence against.
I’d say that secession was a subject for consideration by the courts, but Jefferson Davis thought it was a war-worthy point.
Lincoln was never going to allow this to be settled by the Courts. Lincoln needed the war, and he was going to get it no matter what Jefferson Davis tried to do to prevent it.
The money evidence shows that it is naive to believe the South could have just been allowed to go. Once it threatened the money streams of the connected people, it was only a matter of time before war would be launched to stop it.