I think it's beneficial to see it as: secession (in the US) became definitively illegal because of the Civil War. It was specifically left out of the Constitut

I think it's beneficial to see it as: secession (in the US) became definitively illegal because of the Civil War. It was specifically left out of the Constitution, for a few complex reasons, but from a de facto / de jure perspective, was ambiguous at best, until decided by war.