Now, as Europeans struggle with the social and political strains set off by migration from poor and war-torn nations outside the bloc, some are clamoring to preserve what they feel they never consented to surrender. Their fight with European leaders is exploding over an issue that, perhaps more than any other, exposes the contradiction between the dream of the European Union and the reality of European nations: borders.

Establishment European leaders insist on open borders within the bloc. Free movement is meant to transcend cultural barriers, integrate economies and lubricate the single market. But a growing number of European voters want to sharply limit the arrival of refugees in their countries, which would require closing the borders.

This might seem like a straightforward matter of reconciling internal rules with public demand on the relatively narrow issue of refugees, who are no longer even arriving in great numbers.

But there is a reason that it has brought Europe to the brink, with its most important leader, Chancellor Angela Merkel of Germany, warning of disaster and at risk of losing power. The borders question is really a question of whether Europe can move past traditional notions of the nation-state.