[Old Teufel Hunden]

"Actually your question is backwards - where was the federal government given the right to force a state to remain?"

No it's not. The Southern states could have called a constitutional convention, they could have tried to do so politically. They chose to do it through force (in the article) which is armed rebellion. The rebellion was put down.

[An.American.Expatriate]

The Southern states could have called a constitutional convention, they could have tried to do so politically.

They did not have to. Just as no convention is necessary to grant rights, no convention is necessary to assert them.