This is not ethnic or racial determinism, since the Slavs of southeastern Europe have been shaped politically and economically more by the agency of foreign imperialism than by their own blood and language. The former Byzantine and Ottoman part of Europe — the part closest to the Middle East — is still the poorest, least stable and most in need of support and guidance from the European Union. Whether Europe remains a secure and prosperous continent, or fractures along traditional east-west fault lines — with authoritarians in Russia and Turkey carving out zones of interest — will play out most vividly in the Balkans. Thus, political developments in Paris, Berlin and Brussels have repercussions far afield.

President Vladimir V. Putin of Russia has been active throughout Central and Eastern Europe, and particularly in the Balkans, using various forms of subversion, from running organized crime rings to financing nationalist-populist movements to influencing local news media. Montenegro may be close to joining NATO, but it is often viewed as a veritable colony of Russian oligarchs and crime groups, where by some accounts Russia tried to stage a coup last year. Serbia and Bulgaria are seen as beachheads of Russian regional influence, even as neo-authoritarian governments farther north in Hungary and Poland increasingly bear similarities to the Russian regime. The effort by the Hungarian government to end the freedom of Central European University, founded in Budapest by the Hungarian-American billionaire George Soros after the fall of the Berlin Wall, has to be seen in this geopolitical context.

As for Turkey, President Recep Tayyip Erdogan won a referendum granting him near-dictatorial powers last month. The next day, he visited the tomb not of Mustafa Kemal Ataturk, modern Turkey’s founder, but the tomb of Mehmed II, known as the Conqueror, the 15th-century Ottoman sultan whose imperial armies marched westward from Constantinople as far as Bosnia. Whether it is in Bulgaria, Macedonia, Kosovo or elsewhere in the region, Mr. Erdogan is determined to fill the void opened by a declining European Union. Lawlessness in Macedonia, including violence in the Parliament itself over a contested government transition, demonstrates the political fragility of southeastern Europe.

Yet it is only the European Union that can stabilize the Balkans. Only if Serbia, Albania and Kosovo all become members of the union can the ethnic dispute between Serbs and Albanians truly be solved. Within the European Union, Albania and Kosovo will have no need of unifying on their own. But if they were to attempt unification, it could become a casus belli for the Serbs. A similar dynamic holds for the continuing contest between Croatia and Serbia for influence in Bosnia-Herzegovina. There is peace for everyone in the former Yugoslavia within the framework of the European Union. There is only protracted conflict without it. Indeed, the European Union offers a world of legal states instead of ethnic nations, governed by impersonal laws rather than fiat, where individuals are protected over the group.