Which part of the word "no" do Gerhard Schröder, Germany's chancellor, and Jacques Chirac, the French president, fail to understand? France and the Netherlands have rejected the constitutional treaty: it is dead. But the rejection of the treaty by two of the original six members raises profound questions about the future of Europe and, above all, about the monetary union. A rising tide of integrationist ambition swept the single currency on to the European shore in the 1990s. Now, it is in danger of becoming a beached whale.

Economists argue about the necessary and sufficient conditions for a successful single currency. But the majority would agree that it helps if the area in question is subject to common shocks, markets for goods, services, capital and labour are flexible, the overall economy is dynamic and, not least, there is a shared identity embedded in common political institutions. Not one of these conditions is either necessary or sufficient. But the absence of all four creates a huge challenge. Yet this is precisely where the eurozone now finds itself: economies have diverged; growth is disappointing; markets are proving dysfunctional; and the movement towards further political integration is now in peril.