And the Supreme Court has ruled, in the Texas v White decision, that secession is possible with the consent of the other states. During the rebellion the Southern states seceded unilaterally, which the court found to be illegal.
What rebellion?
Lincoln rebelled against the Constitution!
[...] and [The United States] shall protect each of them [the States] against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence. Figure, Lincoln didn't think this applied - Since he was the one invading......
or
Did he illegally invade a sovereign Country - Which is currently under occupation?