In a 2004 LRC post, How Stupid are Europeans?, I noted that unless an explicit right to secede or exit from the then-proposed European Constitution were added, any countries joining would likely be prevented by force from leaving later. Happily, the EU Constitution was never finally ratified, due to the heroic stubbornness of French and Dutch citizens.
But as noted in Greece Considers Exit from Euro Zone,
It remains unclear whether it would even be legally possible for Greece to depart from the euro zone. Legal experts believe it would also be necessary for the country to split from the European Union entirely in order to abandon the common currency. At the same time, it is questionable whether other members of the currency union would actually refuse to accept a unilateral exit from the euro zone by the government in Athens.
Never join a political union. Never centralize. It could be a one-way ratchet, as the CSA was forced to realize. Decentralization—and the Catholic idea of subsidiarity—is always to be pushed for, down to the individual level.