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

When people refer to “Europe’s crisis” nowadays, they are almost invariably talking about the euro. The reason is obvious enough. Unless European leaders act decisively to stop the currency’s slow rot, it’s likely to disintegrate and take much of the continent’s prosperity with it. But, if they were being honest with themselves, European leaders would admit that the continent’s financial crisis isn’t the only threat to a prosperous future. Europe is in the early stages of a demographic crisis, and it is no more prepared for this slow-moving disaster than it was for the potential collapse of its currency.

The aging and dwindling of much of Europe’s population is the first ingredient of this other, inexorably approaching danger. Some European countries such as the United Kingdom and France have little to worry about; they have long enjoyed strong fertility rates and high levels of immigration, so they will likely have healthy population growth for decades. But many Eastern and Southern European countries—as well as some in Europe’s center, like Germany and Austria—will shrink rapidly. As a result, their economies are likely to stall, and their social safety nets are all but certain to wear thin.

Barring a spectacularly unlikely change in fertility patterns, the only realistic solution for these countries would be massive immigration. But—and this is the second ingredient of Europe’s coming crisis—the countries that face the most rapid population loss are the very countries that have historically been least welcoming to outsiders. Poland, Germany, and Italy, for example, have long granted citizenship on the basis of ethnicity, not place of birth. As a result, many Poles, Germans, and Italians still think that somebody can’t be a true member of their national community unless he hails from the correct bloodline. They remain hostile to the millions of “foreigners” who have lived in their towns and villages for decades, or even generations. For the same reason, they are unlikely to open their borders now.

As a result, Poland, which now has a population of about 39 million people, is likely to lose 7 million inhabitants, or just under a fifth of its population, by 2060. Projections for Germany’s decline are even more pessimistic. In 2003, when the country’s population peaked, close to 83 million people lived there. By 2060, the country will have shed nearly 20 million residents. With up to a quarter of Germany’s population lost to low birthrates, there would then be fewer than 65 million people living in the country.

To be sure, Germany is far from being the world’s most populous country now; it will be far from being the world’s smallest country even once it has lost a quarter of its population. Nor is there any strong reason for European countries to care about their relative size in the first place. Given today’s military technology, for example, it is simply irrelevant how many citizen-soldiers they could send into battle in the unlikely case of war. But it would be wishful thinking to deny the seriousness of Europe’s demographic crisis: Even if total size matters little nowadays, the process of depopulation will have drastic social and economic consequences.

It will, for a start, have a bleak effect on the continent’s age structure. In 1960, 11.5 percent of Germany’s population was 65 or older. By 2060, nearly 33 percent will qualify as senior citizens. Italy is aging just as quickly. In 1960, its population had an average age of 31.2; by 2060, the average Italian will be over 50 years old. A shrinking, increasingly aged population will render Europe’s welfare states untenable in their current form. With more people in need of services and fewer people to foot the bill, the health care and pension systems of many European countries will run out of money, pure and simple.

Perhaps European voters could muster the political will to cut back on these social benefits. As we have seen during the euro crisis, this isn’t likely. But even if they do, a dwindling pool of high-skilled workers would almost certainly lower productivity, increase wage costs, stymie economic growth, and—paradoxically—lead to higher unemployment.

The only realistic way to avoid depopulation’s disastrous effects would be to accept mass immigration. But the levels of immigration that would be required to soften the most serious strains on Europe’s economy are staggering. There are two reasons for this. First, the current population forecasts already assume high levels of immigration. For example, the projection by the German Office for Statistics, which predicts that the country’s population will decline to 65 million by 2060, is based on the assumption that there will be a net influx of 100,000 immigrants a year. (If there were zero net migration, Germany’s population would plummet even more precipitously to 58 million.)

Second, it will be much more difficult for European countries to reach these levels of net migration than it first appears. On average, around 700,000 Germans have left their country each year over the past decade. In other words, Germany would need to welcome more than half a million newcomers every year just to break even with emigration rates. To counterbalance the population loss from its low fertility rate, it would probably need to add more than a million immigrants—every year, for the next 50 years.

So if Germany, Poland, and Italy want to avoid rapid demographic and economic decline, they would have to accept a higher per capita inflow of migrants than the United States for most of the past century. But America was founded as a country of immigrants; one that, whatever the more complicated reality on the ground, has always claimed that an allegiance to the republic’s values was the sole prerequisite for membership. Germany, by contrast, has always defined itself on ethnic lines; even today, many Germans have trouble accepting that somebody who looks black or Turkish could “really” be German. Poles and Italians, meanwhile, have an even more restrictive sense of national identity: Many still believe that a true Pole or Italian must not only be ethnically Polish or Italian but also Catholic.