The third stage in the development of central banks came in 1971 when the US abandoned the Bretton Woods arrangement of fixed exchange rates against the dollar.
Free capital movement and pegged exchange rates are a dangerous mix (unless the currency is fixed under a currency-board arrangement as in Hong Kong, where all local currency must be fully backed by American dollars).
This idea that the euro is a take-it-or-leave-it option, an arrangement of convenience rather than a fixed and irrevocable union, has been a staple for European leaders over the past few years.