Europe’s New Medieval Map. By Robert D. Kaplan.

Amid a wave of migration, Slovenian soldiers set barbed-wire fences on the Slovenian-Croatian border, Nov. 12. PHOTO: AGENCE FRANCE-PRESSE/GETTY IMAGES.

Europe’s New Medieval Map. By Robert D. Kaplan. Wall Street Journal, January 15, 2016.

Kaplan:

As the European Union unravels, the continent is reverting to divisions that go back centuries, writes Robert D. Kaplan.

Look at any map of Europe from the Middle Ages or the early modern era, before the Industrial Revolution, and you will be overwhelmed by its dizzying incoherence—all of those empires, kingdoms, confederations, minor states, “upper” this and “lower” that. It is a picture of a radically fractured world. Today’s Europe is, in effect, returning to such a map.

The decades of peace and prosperity, from the 1950s to 2009, when the European Union’s debt crisis began, made the political and economic contours of the continent look simple. There were two coherent blocs for the duration of the Cold War, and they were succeeded by the post-Cold War dream of a united Europe with its single currency. Today, as the European Union suffers one blow after another from within and without, history is reversing course—toward a debilitating complexity, as if the past half-century were just an interregnum before a return to fear and conflict.

For the U.S., the reality of this new situation is only just now coming into view. Europe, whose economy rivals that of the U.S. as the largest in the world, remains an asset and an ally, but it is also a profound problem. The pressing question is how to manage it.

Europe’s divisions were visible for decades as the EU worked to expand its boundaries and practical reach. There were those countries inside the EU and those outside; those inside the borderless zone of free travel (the Schengen Area) and those outside; those able to manage the financial rigors of the eurozone and those unable to do so.

What is less appreciated is the deep roots of these divisions in the continent’s history and geography. The sturdy core of modern Europe approximates in large measure the Carolingian Empire founded by Charlemagne in the ninth century. The first Holy Roman Emperor, he ruled the lands from the North Sea down through the Low Countries and radiating outward to Frankfurt, Paris, Milan and so on. The weaker cousins of this Europe extend along the Mediterranean, from the Iberian peninsula to southern Italy and the historically less-developed Balkans, heirs to the Byzantine and Ottoman traditions.

Charlemagne receiving the submission of the Saxon leader Witikind at Paderborn in 785. By Ary Scheffer (1795-1858). Versailles.

During the decades following World War II, this divide was suppressed because of Europe’s relative isolation from its “near abroad”—that is, from the regions of North Africa and Eurasia that, for centuries, did so much to shape the distinctive character of the continent’s periphery. Today that wider geography can no longer be ignored, as Europe’s various regions adopt very different attitudes to the threats posed by Russia’s bullying under President Vladimir Putin, the flood of refugees from the Middle East and the latest terrorist outrages at home and abroad. It has become clear that the centralization imposed for decades by the EU and its distant, unrepresentative bureaucracy hasn’t created a unitary Europe. Indeed, it has created a powerful backlash across the continent, one that the EU can survive only by figuring out how better to establish its legitimacy among its diverse nations.

The geographical defenses that shielded Europe during the postwar era no longer hold. When the great mid-20th-century French geographer Fernand Braudel wrote his classic work on the Mediterranean, he didn’t treat the sea itself as Europe’s southern border. That, he suggested, was the Sahara. Today, as if to prove him right, migrant caravans assemble across North Africa, from Algeria to Libya, for the demographic invasion of Europe proper. The Balkans, too, have resumed their historic role as a corridor of mass migration toward Europe’s center, the first stop for millions of refugees fleeing the collapsed regimes of Iraq and Syria.

Europe thus now finds itself facing an unhappy historical irony: The decades in which it was able to develop its high ideals of universal human rights, including the right of the distressed to seek havens in Europe, was made possible, it is now clear, by the oppressive regimes that once held sway on its periphery. The Arab world was slammed shut for decades by prison states whose dictator-wardens kept their people in order. Saddam Hussein in Iraq, the Assad family in Syria, Muammar Qaddafi in Libya—they allowed Europe to have its idealistic cake and eat it, too.

Worse for European unity, geography and history have conspired to make some regions of the continent more vulnerable to the flood of migrants and refugees than others. As Germany and parts of Scandinavia lay down a very tentative welcome mat, Central European countries like Hungary and Slovenia erect new razor-wire fences. The Balkans, virtually separated from the rest of Europe by war and underdevelopment in the 1990s, have now been dealt another blow by the anarchy in the Middle East. At the southeastern extremity of Europe, Greece, once a poor Ottoman province, has seen its economic crisis exacerbated by its unlucky position as the gateway for hundreds of thousands of migrants fleeing the Arab world’s turmoil.

Another critical factor in the period of relative stability now coming to an end in Europe was the geopolitical role played by Russia. During the Cold War, the Soviet Union was an obvious strategic threat, but it was a threat well-managed by the U.S., and for most of the period, after Stalin’s demise, the Kremlin was led by stodgy, risk-averse functionaries. After the Soviet collapse, a decade of turmoil and institutional weakness in Russia meant, among other things, that it was no threat to Europe.

Today, needless to say, Russia is very much back as a strategic player in Europe. Mr. Putin’s consolidation of control inside Russia following the infirmity of the Boris Yeltsin era has created a deep divide between Paris and Warsaw, Berlin and Bucharest. If you were a Pole or a Romanian in the 1990s, Russia was conveniently weak and chaotic, and membership in NATO and the EU held out the prospect of lasting peace and prosperity. The strategic horizon is very different now: The future of the European enterprise appears uncertain, and a revived Russia has annexed Crimea, overrun eastern Ukraine and again threatens your own borders.

Here we may be witnessing the start of a remarkable reversal of Cold War alliances. Europe is again redividing into halves, but this time it is Eastern Europe that wants to draw closer to the U.S. because it increasingly doubts that NATO alone will be an effective defensive barrier against Russia. Meanwhile, the countries of Western Europe, worried about the tide of refugees and terrorist attacks at home, seek to draw closer to Russia (the Ukraine crisis notwithstanding) as a hedge against the chaos emanating from Syria.

Mr. Putin knows that geography and raw power—both military and economic—are still the starting point for asserting national interests. Europe’s elites take a very different view. After centuries of bloodshed, they have largely rejected traditional power politics. To maintain peace, they have instead placed their hopes on a regulatory regime run by the post-national technocrats of Brussels. In their minds, the continent’s divisions could be healed by the social-welfare state and a common currency. Distinctive national identities shaped by centuries of historical and cultural experience would have to give way to the European superstate, whatever its toll on the political legitimacy of the EU among the diverse nations of Europe.

In the U.K. and much of Western Europe, there is now a backlash against the overreaching of Brussels, and it is finding powerful expression in domestic politics. Social-welfare policies once touted as a balm for the continent’s divisions have acted as a drag on national economies, and this stagnation has provided, in turn, the backdrop for nationalist (sometimes reactionary) politics and rising hostility to refugees.

Still another set of concerns is visible in Central and Eastern Europe. For the past three years, I have been traveling back and forth to Romania, a country where World War II ended only in 1989, with the downfall of the Stalinist Ceausescu regime. In Romania, as in the Baltic states and other parts of the former Warsaw Pact and Soviet Union, the EU still represents more than a balance sheet. It stands for a politics based on modern states rather than on ethnic nations, governed by the rule of law rather than by arbitrary fiat, protecting individuals no matter their ethnic or religious group, or their father’s name.

The region from the Baltic states and Poland, south to Romania and Bulgaria, and then east to the Caucasus constitutes what I call the Greater Intermarium (Latin for “between the seas,” in this case, between the Baltic and Black). The Intermarium was a concept invented by Josef Pilsudski, the Polish leader of the 1920s and 1930s, who hoped to see a belt of sturdy democracies between Germany and the Soviet Union to thwart the imperial tendencies of both.

The threat today, of course, is solely from Russia and not from Germany. Germany’s political dominance of Europe should flow naturally from its economic dominance, and that has happened to some degree, with power moving east from Brussels to Berlin. But German leadership remains awkward and hesitant. Of all the European elites, Germany’s in particular have, since the late 1940s, put their faith in European integration, in large part as a way to exorcise the demons of their own past.

In the face of multiple crises, Chancellor Angela Merkel has played a deft political hand, with only occasional setbacks like the recent news of sexual assaults committed on New Year’s Eve by Arab migrants. But Ms. Merkel is no Bismarck or Frederick the Great, nor would she want to be. The legacy of Nazism and the ambivalence of sitting between the West and Russia weigh heavily against German leadership.

As the EU continues to fracture, this power vacuum could create a 21st-century equivalent of the late Holy Roman Empire: a rambling, multiethnic configuration that was an empire in name but not in fact, until its final dissolution in 1806.

This means that there is still no alternative to American leadership in Europe. For the U.S., a Europe that continues to fracture internally and to dissolve externally into the fluid geography of Northern Africa and Eurasia would constitute the greatest foreign-policy disaster since World War II. The success of the EU over many decades was a product of American power, stemming from the victory over Nazi Germany. For all its imperfections, the EU, even more than NATO, has been the institutional embodiment of a postwar Europe that is free, united and prosperous.

Elements of the Obama administration, to their credit, have tried valiantly to grapple with Europe’s post-Cold War disintegration. The Pentagon has put forth plans for the return of more ground troops, and Victoria Nuland, the assistant secretary of state for European affairs, has been energetic in standing up to Russia in Ukraine.

But President Barack Obama himself has evinced a certain lack of interest in the continent’s travails and has taken a less than robust posture toward meeting Mr. Putin’s aggression. The administration is plainly distracted, its attentions focused on crises not only in the Middle East but in the Pacific Basin as well. The problem is not, however, the president’s much-discussed “pivot to Asia,” where U.S. leadership is also very much needed to rally our allies. The problem is the mistaken idea that somehow Europe matters less than it did during the Cold War.

The current administration and its successor must put the security of the Greater Intermarium at the center of its priorities. This is a matter not just of more military aid but of more robust diplomatic engagement with every country from the Baltic to the Black seas. The aim should be not just to resist Putin’s aggression but to maintain the internal cohesion and capacity of both the EU and NATO.

At the political level, this will mean helping the EU to develop in a direction that provides more democratic accountability. As for security matters, a turn to Europe will mean putting an end to the counterproductive view that the U.S. will do more for Europe’s defense only if NATO member states themselves raise their defense budgets. With a few exceptions, that isn’t going to happen amid today’s economic woes. If Europeans were to see greatly intensified U.S. involvement, however, they would be more likely to take bold actions to save their own institutions.

The decades when we thought of Europe as stable, predictable and dull are over. The continent’s map is becoming medieval again, if not yet in its boundaries then at least in its political attitudes and allegiances. The question today is whether the EU can still hope to permanently replace the multicultural Habsburg Empire, which for centuries sprawled across Central and Eastern Europe and sheltered its various minorities and interests.

The answer will depend not only on what Europe itself does but also on what the U.S. chooses to do. Geography is a challenge, not a fate.