THIS summer, a striking, often tragic story has been playing itself out on the outskirts of Calais in France, at the entrance to the tunnel that connects the European mainland to Great Britain. Thousands of migrants, African and Middle Eastern, have been trying to sneak onto the trucks and trains that traverse the tunnel, cutting through wire fences and evading the police along the way. Ten have died, but enough have succeeded for many more to keep trying, while politicians on both sides of the Channel point fingers and a refugee camp outside Calais remains swollen with would-be subjects of Elizabeth II.

In certain ways this crisis resembles last summer’s border surge in the United States, when a wave of juvenile migrants overwhelmed the border patrol’s ability to cope. But mostly Calais highlights two major differences between the immigration issue in America and Europe, two ways in which migration — from Africa, above all — is poised to divide and reshape the European continent in ways that go far beyond anything the United States is likely to experience.

The first difference is illustrated by the Calais migrants’ desire to reach not only the European Union at large but the specific destination of Britain — because of its relatively stronger economy, because they speak English, because the U.K. doesn’t have a national identity card, or for other reasons still.

An immigrant desire to go further up and further in is entirely normal. (Mexican immigrants to the United States do not all settle down in El Paso or Tucson.) But it poses a major dilemma for the European Union, which allows free movement across its internal borders, but which is composed of nation-states that still want sovereignty over their respective immigration policies.

America has a mild version of this tension: Witness the recent debate over “sanctuary cities,” or state-federal conflicts over immigration enforcement. But it’s understood that immigration policy is ultimately set nationally. Michigan isn’t going to close its borders and impose identity-card checks; Maine isn’t going to secede in order to set its own immigration quotas.

But in a less-united and federalized Europe, the desire for real national control over immigration policy may be as dangerous to the E.U. project in the long run as the already-evident folly of expanding the common currency to Greece.

Already the desire for immigration sovereignty is behind Britain’s possible referendum on a “Brexit” from the European Union. It’s behind Denmark’s experiment with reimposing border controls. It’s behind the rise of the National Front in France, and Euroskeptical parties the continent over. It’s adding to Europe’s already-significant north-south divisions, since (poorer) southern European countries are receiving the bulk of recent migrants and (richer) northern European countries would prefer the new arrivals remain in Italy or Spain.

And these pressures are only likely to increase, because of the second difference between immigration in Europe and America: Namely, the scale of the migration that may be coming to Europe over the next fifty years.

That scale could be set by the staggering growth of Africa’s population, and the native European population’s stagnation and decline. Consider how much Latin American immigration has roiled U.S. politics (hello, Mr. Trump), when there are just over 300 million people in the United States and just under 600 million in all the countries to our south — a ratio that’s unlikely to change much over the next few generations.

Then consider: Today there are 738 million Europeans (500 million of them in the E.U.) and just under 1.2 billion Africans. In 2050, according to the latest U.N. projections, Europe’s population will have dipped to (an aging) 707 million, while Africa’s population will be 2.4 billion. By 2100, there will be 4.4 billion Africans – two of every five human beings overall — and Europe’s population will be just 646 million.

The Mediterranean is far wider than the Rio Grande, but this is still a wildly unstable demographic equilibrium. What’s more, as Noah Millman pointed out recently in Politico, northward migration – a kind of African “scramble for Europe” – is likely to increase whether African states thrive over this period or collapse. Desperation might drive it, but so might rising expectations, the connections forged by growth and globalization. (Many Africans currently braving the Mediterranean, for instance, seem to be ambitious, educated citizens from countries with growing economies, not just refugees.)

If Africans were to migrate to Europe at the rate Mexicans have migrated to America since 1970, Millman notes, by 2050 a quarter of Europe’s population would be African-born. That probably won’t happen: The birthrate projections will be off, the migration patterns will be different, European countries will impose restrictions that actually succeed in keeping people out.

But something significant is going to happen. In some form, a Eurafrican future is on its way. And judging by the stumbling response to a few thousand migrants at Calais, Europe is deeply unprepared.