(Thirteenth in series: The interpretation of statutes and the denial of judges' powers.)
The obvious reason to think secession is Constitutionalism's weakest point is that's where Constitutionalism almost broke. The Federal government won the Civil War militarily and morally, but neither victory meant the North had the better legal theory. Secessionist movements continue to lurk at the social margins because Constitutionalism's logic favors the right of states to secede.
The U.S. Constitution contains vertical as well as horizontal checks and balances, the federal government balanced by the states. The federal government can preempt the states in distinct domains, but outside, the States have exclusive jurisdiction. Yet, the system is ultimately unbalanced, in that a branch of the federal government resolves conflicts between the states and the federal government. Unlike other checks, where each evokes a potential countercheck, the states can't answer an adverse decision by the U.S. Supreme Court. The Supreme Court may be sympathetic to states' rights, but the long-run trend is in the opposite direction to the extent the structural factor determines the drift.
The federal government continually gains power at the states' expense. To note this trend is not to extol states' rights or to maintain they beneficially check the federal government but only to mention a consequence of structural imbalance; nor is it to say that the imbalance — the structural deviation from blueprint — lacks adverse consequences. Missing for a true system of mutual checks is the necessary counterweight in a system where states' rights are supposed to endure: the right of the states to secede. That secessionism is simultaneously inimical to the Constitutional system points to Constitutionalism's peril, not its salvation, but secessionist movements' failure also shows that for general historical determinism legal structure isn't omnipotent.