You have a rather simplistic view of things. Both sides were hypocrites. At the beginning, the South said "We Secede." The North said "You can't secede." When the war was over, the South said, "Okay, since we can't secede, we will send our Representatives and Senators back to Congress. The North said "No. You Seceded. "
Did they secede, or didn't they? I guess when the US wanted to regard them as having seceded, they did, and when it wanted to regard them as not having seceded, they did that as well.
At the beginning, the South said "We Secede." The North said "You can't secede." When the war was over, the South said, "Okay, since we can't secede, we will send our Representatives and Senators back to Congress. The North said "No. You Rebelled."
They did not secede. They committed insurrection. After the insurrection, they had to be readmitted, just like any territory.
Is this too hard for you? Too many big words?