Why Jewish Poles, black Germans, and headscarf-wearing Italians are the continent’s best hope.

As the rapid rise of European right-wing populism has shown in the last 20 years, this mix between a new reality of mass immigration and an ongoing devotion to the old idea of a nation defined by a common ethnicity or religion is potentially toxic. What do rising levels of anti-immigrant sentiment augur for a future in which immigration is likely to tick up even as living standards are likely to decline?

In essence, there are three possible scenarios. In the first, widespread anti-immigrant sentiment, coupled with the rising power of populists, stops many European governments from opening their borders further. Though voters are likely to delude themselves about this bitter truth for a time, this would result in a half-century of economic stagnation and a radical dismantling of the European welfare state.

In the second scenario, European leaders manage to bully their deeply reluctant populations into accepting more immigrants. This would create the potential for renewed economic growth, and might just rescue the continent’s pension and health care systems. But if streams of outsiders enter Germany, Italy, and Poland, even as local populations cling to their exclusionary definition of citizenship, the social and cultural consequences could be dangerous. There would then forever be two classes: a dwindling majority of resentful “natives” and a growing minority of underprivileged “foreigners.” At best, it would be a volatile combination.

In the third, most optimistic, scenario, Germany, Italy, and Poland would follow the American and Canadian models, and reinvent themselves as real immigrant destinations. Naturally, they would expect newcomers to obey the laws of a free and democratic society. But they would also grow to accept that there can be such a thing as a Jewish Pole, a black German, or a headscarf-wearing Italian.

The last scenario is probably the least likely. But if those European countries whose populations are set to dwindle rapidly over the coming decades want to avoid the twin dangers of economic collapse and civic strife, they have no choice but to turn themselves into multiethnic societies—in ethos as well as fact. If they don’t, the fact that they may no longer share a common currency will be the least of their worries.