When Mr. Erdogan’s Justice and Development Party, known by the Turkish initials AKP, came to power in 2002, the EU failed to grasp the nature of the party’s limited mandate. Turkey’s high election threshold bars parties earning less than 10% of the national vote from Parliament and distributes their seats to larger parties. By winning a little more than a third of the popular vote, the AKP received two-thirds of the seats. In subsequent elections, even with strong economic growth, the AKP has never received more than 50% of the vote. But the EU has consistently failed to engage and empower Mr. Erdogan’s opposition, which comprises roughly half the population and includes liberal, leftist and secular Turks.

The EU has also failed to appreciate the true nature of Mr. Erdogan’s political movement. The AKP is rooted in political Islam, a nativist and illiberal force, not unlike the far-right parties that have recently emerged on the Continent. In the past decade, however, the EU turned to Mr. Erdogan to consolidate Turkey’s liberal democracy. In 2005, Brussels opened membership talks with Ankara, putting Mr. Erdogan in charge of taking Turkey into Europe.

Mr. Erdogan embraced EU accession tactically in order to defang secularist Turkish generals. The military, which once saw itself as the grand arbiter of Turkish society, often interfered in politics, undermining parties rooted in political Islam such as the AKP. The Eurocrats told Ankara it needed to “uphold the rule of law,” which meant the EU wanted the Turkish generals out of politics. Mr. Erdogan happily obliged, carrying out reforms to eliminate the military’s political role, thereby abolishing his nemesis.

The EU was right to insist that the military exit politics in order to consolidate Turkey’s democracy, but wrong to assign this task to an illiberal movement. Between 2007 and 2010 I frequently spoke with high-level EU officials who seemed to think the military was the main obstacle to democracy in Turkey. With the generals out of politics, they said, things would fall into place.

Developments have proved them completely wrong. Once he neutralized the generals, Mr. Erdogan no longer felt it necessary to please Brussels. And Brussels, after forcing the military out of Mr. Erdogan’s way, didn’t take up its role as the new grand arbiter of Turkish democracy. Germany and France objected to Turkey’s full membership, and Brussels began backtracking on the idea of Turkish accession.

Disaster ensued. With help from followers of Fethullah Gulen, his close ally at the time, Mr. Erdogan built a 2008 court case alleging that a secret cabal of secularist generals was responsible for numerous attempts to sabotage and destabilize the Turkish government. Prosecutors never proved their case, but Mr. Erdogan used the proceedings as a pretense to wiretap opponents, investigate media outlets, and jail journalists. Freedom indexes, which had previously reflected improvements in Turkish democracy, began to falter and slide. Following a set of trials collectively dubbed “Ergenekon,” Mr. Erdogan changed the country’s constitution, granting himself the power to appoint high-court judges without a confirmation process.

The failed coup in 2016 was a genuine attempt to oust Mr. Erdogan on the part of military officers, at least some of whom appear to have been aligned with the Gulen movement. After the coup attempt, however, Mr. Erdogan did more than target the plotters. He used his emergency powers to carry out a broad crackdown on political opponents who had nothing to do with the coup.

But Brussels dropped the ball long before 2016. Between 2002 and 2012 record amounts of European foreign direct investment flowed into Turkey in anticipation of EU accession. Turkey’s economy has grown at a brisk pace in the past decade, a rising tide the AKP has ridden to repeat electoral victories. If Brussels had pulled the plug on accession talks before 2012, Mr. Erdogan would have been forced to change his ways.

The EU’s leverage over Ankara has weakened following the eurozone crisis. Now the EU finds itself bending over backward to please Mr. Erdogan. French President Emmanuel Macron recently invited Mr. Erdogan for a state visit. The new sultan will be honored with a dinner at the Élysée Palace.

Mr. Macron seeks Turkey’s assistance on terrorism and immigration, two issues of great importance to French voters. Mr. Erdogan’s government is doing an excellent job of blocking refugee flows and preventing the return to Europe of foreign fighters from Syria and Iraq. European leaders won’t alienate him anytime soon. German Chancellor Angela Merkel is slated to visit Turkey in the coming months, which could bolster Mr. Erdogan’s domestic image as a statesman should he decide to call those snap elections.

The Turkish people are ultimately responsible for the health and survival of their own democracy. But it’s high time Europe accepted its share of responsibility for enabling Mr. Erdogan and encouraging his most illiberal instincts.