I think the best way to understand the Civil War is by the principle of separation of powers of Montesquieu. I mean to say that even though this principle is used in understanding of the separate branches of government (Executive Judicial, Legislative), I think the exact same principle applies to why the founding fathers gave only limited powers to the Federal government and all other powers were reserved for the States.
So they saw the States as essentially in the same sort of category as a separate branch of government that had all other powers that were not specifically given to the Congress or the other branches of Federal government.
So the question was if the Federal government was given in the Constitution the power to prevent a state from leaving the Union. Since it would be hard to make such a case, Robert E Lee concluded that the South was right.
[I have heard other arguments but they do not seem to carry much weight. especially once the South a]saw that the North was violating the Supreme Court's ruling about runaway slaves, it was clear the north was actively violating the Constitution.]
