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. And that is a question that Europeans have avoided confronting, much less answering, for over half a century.

Backpedaling on Open Borders

In 2015, at the height of the refugee crisis, Ms. Merkel warned that if European countries did not “fairly” share the burden, then opportunistic leaders could exploit the issue to dismantle Europe’s freedom of internal movement. “It won’t be the Europe we want,” she said.

Three years later, Ms. Merkel has become the leader she warned about. To save her governing coalition in Berlin and bleed off populist sentiment, she has proposed imposing controls at the Austrian border to block refugees.

Most refugees arrive in Italy, Greece or Spain and are meant to remain there while waiting for asylum. In practice, though, many head north.

But how to pick out refugees from dozens of open roads and rail lines that connect Germany with Austria?

One option is to screen selectively for possible refugees; in essence, racial profiling. No one is sure how this would work. Spotters on border watchtowers with binoculars? Random pullovers? Any scheme seems likely to miss most refugees while harassing enough dark-skinned non-refugees to guarantee a backlash.

The other option is to set up checkpoints and screen everyone, making travel from Austria to Germany far more difficult, likely hurting both economies. In either scheme, Ms. Merkel’s plan calls for camps along the border to hold refugees seeking to cross.