In 2012, the ECB faced down a mortal threat to the euro: fears of redenomination (the re-introduction of domestic currencies) were feeding a run away from banks in the geographic periphery of the euro area and into German banks. Since President Mario Draghi spoke in London that July, the ECB has done things that once seemed unimaginable, helping to support the euro and secure price stability.

So far, it has been enough. But can the ECB really do “whatever it takes”? Ultimately, monetary stability requires political support. Without fiscal cooperation, no central bank can maintain the value of its currency. In a monetary union, stability also requires a modicum of cooperation among governments.

Recent developments in France have revived concerns about redenomination risk and the future of the euro....