You are at least partly wrong about the treason.
At the time of the civil war, the federal government was nowhere near as strong as it is now, and states were far more powerful politically. (Lincoln had non-trivial struggles with the governors of various Union states, in raising troops and revenue. Bruce Catton did some nice writing on that in his various civil war books). Since the Constitution didn't preclude secession, in explicit language, at the time, it was not unreasonable for the states to take a Ninth and Tenth Amendment position and declare that they had reserved the right to secede. (The court case people often refer to as having more recently settled the matter of how legal secession is now is a post Civil War decision). That said, Lincoln was right to protect Federal property, being the head of the executive branch of the Federal Government, and so we see Fort Sumter ... Harpers Ferry ... and so on.
The Civil War was in many ways a Constitutional test, but I'll not derail into that, you can save it for the history sub forum.
DR