So I'm wondering, do the States actually owe the Fed money for the National Debt? Lets say in 5-10 years the dollar completely tanks - would it be feasible for a state or group of states to leave the Union and set up their own currency without bringing along a chunk of the debt rung up by Federal policies? I think that kind of dissolution of the Union could actually work, and may even be likely - and unlike the Civil War, Uncle Sam would be hard pressed to field troops with a worthless dollar and few men thinking it was something worth fighting for.
Would it be feasible? No. Every state is utterly dependent on the Federal government to meet budgetary requirements for things like roads and medicaid. Is there any law or governmental body to enforce debt collection? No. But even if the dollar was worthless, they'd just field an army by letting them loot from the conquered.
If you reach that sort of situation what is technically legal or not becomes irrelevant and it turns into who has the biggest stick.
They wouldn't even need to leave the Union. They'd just pass a law nullifying the legal tender laws regarding Federal Reserve Notes. Of course, if it's California or Hawaii, they'd probably suffer an invasion from China as a result.