"After 620,000 had perished, the issue of a state's right to secede was settled at Appomattox. If that right had existed, it no longer did."
You're confusing might with right. Might does not make right in this case, it just makes for tyranny. The right still exists, just as it existed in April, 1865. The might still rests with those in control of the "Federal" government. Secession won't happen in modern times because very, very few are willing to die for liberty in this day and age.