From time to time, national referendums have thrown a spanner in the European Union's plans for closer ties between its members. But never has there been such a broad renunciation of that process as that delivered in the recent European Parliament elections. In an alarming number of the EU's 28 member states, populist parties from the far right and far left triumphed over their mainstream opponents.
The impact was most notable in Britain, where the UK Independence Party topped the poll with 28 per cent of the vote, and France, where the anti-European National Front did likewise with 25 per cent support. Centrist pro-European parties will continue to be the dominant force in Brussels, but this is not an outcome that can be shrugged off.
It is clear that after 60 years, during which the EU and its forebears have, by and large, orchestrated peace and prosperity, many of its 500 million people have fallen out of love with the pan-Europe ideology.
They complain about the arrogance and expense of bureaucrats in Brussels who are intent on reducing the important of their national parliament. They regret replacing their national currencies with the euro, which, rather than making Europe more equal, has created instability. And those in the north decry an expansion that has saddled them with indebted nations in southern Europe. The EU has, says David Cameron, the British Prime Minister, become "too big, too bossy and too interfering".
This sentiment has transformed the fortunes of parties such as Ukip, which for the vast majority of its two decades of existence, has been the most peripheral of fringe parties. But the problem for the eurosceptics who will gather in Brussels is that as much separates as unites them. Ukip, for example, wants nothing to do with radical anti-immigrant stance of far-right French and Dutch parties. There are also different views of what should happen next. Some want to withdraw their countries from the EU. That is the policy of Ukip and the National Front, which wants the repatriation of all powers to Paris, the end of the euro, and an end to free movement within Europe. Others, however, believe the EU can be saved by reform.
The latter course can prevail if the European Parliament heeds the unmistakable lesson of this election and puts a brake on the drive towards ever closer union. It needs also to be less intrusive in the everyday affairs of its members. Equally, it must convince Europeans that it provides the framework to outperform other developed countries economically. The most convincing answer to the eurosceptics lies, as Germany's Angela Merkel suggested, in "improving competitiveness on growth and creating jobs". At some point, those countries using the euro must also embrace a more comprehensive fiscal union. If that is not done, a return to national currencies is the logical step.
The economic tide is swinging in favour of the pro-Europeans. Much of the EU has been late to catch the global upswing, but even the weaker economies are starting to benefit. They will gain also from the tough measures taken over the past few years. Further, the conclusion of a successful free-trade pact with the United States would hammer home the message that union can deliver more wealth than individual endeavour.
The global community should hope for that outcome. It is easy to forget the EU's greatest achievement, that of delivering peace to a continent scarred previously by never-ending conflict. Voters have indicated their national identities must remain important. That, however, is no reason to disown the reasoning that created a united Europe.