The largest unresolved issue in the European Union (EU) is mobility. The integration process was intended to make it easier and more attractive for Europeans to move from one country to another. According to this vision, the EU's inhabitants had the nation to lose and a continent to gain. But some recent election results indicate that they are more worried about losing the nation.
Ever since the 1986 Single European Act removed restrictions on working in other member countries, the continent has been a single labour market - at least in theory. It was a policy that fitted well with other parts of the integration agenda. The euro's ability to function as a common currency would require a flexible labour market in which workers could adjust to regional shocks by moving.
But it was only after the global financial crisis that European migration really took off. And the result has been a backlash against it, first in destination countries, and now, as Poland's just-concluded presidential election suggests, in countries of origin.
As the debate over migration has heated up, both those who moved and those who stayed behind have ended up feeling more nationalistic, not more European.
At first glance, the results of recent elections in France and England might give the impression that European migration is on the wane as a hot-button issue.
The French National Front did badly in regional polls compared with the mainstream and pro-European right. It then descended into scandal and acrimonious infighting between the party leader Marine Le Pen and its founder, her father, Mr Jean-Marie Le Pen (who was ultimately expelled).
Meanwhile, Britain's anti-European United Kingdom Independence Party performed poorly in the election last month, and then fractured. Germany's right-wing populist Alternative for Germany is also splintering and disintegrating. Italy's Five Star Movement is in abeyance as well.
This sequence of failures has led some to predict the end of protest politics. That view is not quite right. The far-right political parties that seemed to be the euro crisis' main beneficiaries may have stumbled. But the issue of labour mobility is not going away.
Until now, the discussion about European migration has taken place mostly within rich destination countries, like France and Britain. Little attention has been given to its effects in the countries of origin.
But in Poland's presidential election, the issue was front and centre, where it generated the greatest resonance with voters and contributed to the victory of Mr Andrzej Duda, head of the right-wing Law and Justice Party. It also provided the theme for the campaign of rock musician Pawel Kukiz, who finished third in the election's first round.
Both of them complained that too many young Poles were leaving the country and that their country was becoming an empty land of the elderly - a fact that is obvious to anyone who walks down a Polish street. Europe might still be home to dynamic young Polish communities, but they are in London, Dublin, Paris, Oslo and Stockholm, not in Warsaw, Krakow or Lodz. And they are certainly not in the poor, rural communities in the east and south of the country, which have been disproportionately emptied by emigration.
Debate about the damage done by emigration has also erupted in countries much more obviously affected by the economic crisis - eurozone countries like Greece, Spain and Portugal, and countries outside the currency union, like Bulgaria and Romania.
And, though the issue raises obvious questions of justice and efficiency - poor countries, having invested large sums in education, now produce graduates who take jobs and pay taxes abroad - Europe has provided little in the way of an effective response.
True, some migrants might eventually return, bringing skills and capital with them. But, in the meantime, they are leaving behind a population that is older, poorer and more vulnerable. European solutions to the problem could involve transfers of funds when local education benefits a continent-wide labour market or help in addressing the problems that affect areas at risk of losing their working-age populations.
The irony is obvious. European integration has sparked increased mobility - especially in the aftermath of the euro crisis. But what it has not yet done is generate the institutional framework needed to make mobility acceptable to the EU's residents.
The writer is professor of history and international affairs at Princeton University and professor of history at the European University Institute in Florence, Italy.