Yes, that's what happens in rebellion. And rebellion is not a "lawful" undertaking anywhere.
Rebellion, or attempted revolution, is the right of any community that feels itself pushed to extreme necessity, but it only becomes lawful if it succeeds. The American Revolution succeeded, the Confederate one didn't.
It is entirely understandable that the rebelling Confederates should seize deeded Federal property as part of their rebellion, but, it was an extra-legal action. And for us, today, to say they had a legal right to do so is absurd. They had the right, as a discrete segment of the population, to attempt revolution, as the Colonists had in the 1770s, but the Federal government had every lawful right to resist, as the British authorities had. Today, there is no "blame" to be put, except upon the secessionists' justifications for their rebellion, and their arrogant mis-judgement as to the ability and willingness of the people of the United States to resist their claims.