After a tirade of tweets condemning the House impeachment hearings on the future of his Presidency, Donald Trump spent the rest of Wednesday trying to cajole his Turkish counterpart, Recep Tayyip Erdoğan, into coöperating—on anything. “We’ve been friends for a long time,” Trump told reporters, with the Turkish leader at his side, in the Oval Office. “I’m a big fan of the President,” he said later, at their joint press conference. Never mind that Trump repeatedly mangled Erdoğan’s name, as he also did during Erdoğan’s last visit, in 2017, albeit mispronouncing it in different ways.

Trump made no tangible headway, even though he dangled the bait of a trade deal worth a hundred billion dollars. The President’s latest foreign-policy flop was not all of his making, however. Under Erdoğan, Turkey has become increasingly brazen in defying the United States, the West, and even the NATO alliance to which it contributes the second-largest force.

In recent years, U.S. officials have complained that Turkey allowed jihadis to slip across its southern border to join ISIS, an Al Qaeda faction, and other militant groups in Syria. Turkey then invaded Syria, this fall, to fight the U.S.-backed Kurdish militia that defeated ISIS. Turkey has also cozied up to Russia militarily. It has coöperated with Iran, and a state-owned bank facilitated a multibillion-dollar Iranian scheme to evade U.S. sanctions. At home, Erdoğan has cracked down on political opponents, the media, business leaders, academics, and even his own military to consolidate his rule. Thousands have been arrested in violation of the human-rights principles of the European Union, which Turkey long sought to join.

“It is fair to ask if Turkey is still really an ally of the United States in anything more than name,” Phil Gordon, a senior fellow at the Council on Foreign Relations who served as the Obama Administration’s White House coördinator on Middle East policy, told me. “Ten years ago, we saw Turkey as a country moving toward the United States and the West, reforming and growing economically, democratizing and getting the military out of politics, seeking to join the European Union, and working with the United States in NATO from Afghanistan to the Balkans.” But in the past few years, he said, Turkey has moved in the opposite direction. “Turkey remains an important potential partner of the United States, but the days when the United States aspired to a ‘model partnership’ based on common values and interests are over.”

The crisis is the worst in the modern history of relations between Washington and Ankara, Gönül Tol, the director of Turkish studies at the Middle East Institute, in Washington, told me. “Anti-Americanism has always been there, but it has peaked in the last few years,” she said. “An overwhelming majority of people in Turkey think that Russia is a better ally, and that the bigger national-security threat comes from the U.S.”

Turkey and the United States have survived past crises, including tensions over Cyprus in the nineteen-sixties and seventies; the U.S. rejection of Turkey’s arms requests in the nineteen-nineties, citing human-rights abuses; and the refusal by Turkey’s Parliament to let U.S. forces invade Iraq from Turkish soil in 2003. In the past, there were constituencies in Turkey—through military-to-military ties and center-right politicians—that helped mend ties. “Both constituencies are not there anymore,” Tol said.

Since a coup attempt against Erdoğan, in 2016, the military has become more ideological. “It’s very difficult to promote closer U.S.-Turkey ties or NATO-Turkey ties,” Tol said. Tens of thousands have been purged from the military. “If you are seen as pro-NATO now, it could kill your career.” In turn, she said, the Pentagon—especially Central Command, which operates in the Middle East—has grown wary of the Turkish military because the personnel emphasis is on ideology and religious views more than on professionalism.

Since the nineteen-sixties, the two nations—which represent the western and eastern flanks of NATO, the world’s largest military alliance—usually shared views on common threats, whether it was the Soviet Union or extremism. No longer. One of the most contentious disputes is Erdoğan’s decision to buy advanced S-400 missiles—an air-defense system—from Russia. The first missile batteries, which were part of a two-and-a half-billion-dollar deal, were delivered in July; the second delivery was made in September. The U.S. countered by barring Turkey from manufacturing or purchasing advanced F-35 warplanes. (The U.S. fears that the Russian air-defense system could allow Russia to gain access to F-35 communications and defenses if they operated in the same theatre.) No other NATO member buys Russian military equipment, in no small part because the alliance was established to counter Moscow’s influence.

By law, the White House is also mandated—under the Countering America’s Adversaries Through Sanctions Act, from 2017—to impose sanctions on countries that acquire Russian defense equipment. Trump has not yet imposed sanctions, on the grounds that the system will not be operational until April. At his press conference with Erdoğan, Trump acknowledged that Turkey’s acquisition of the S-400s had created “very serious challenges.” The leaders had failed to resolve their differences, he said, but would continue to talk. Washington wants Ankara, at a minimum, to further defer launching the system. Experts think that Erdoğan is unlikely to cave.

The differences between the erstwhile allies are now broad. “The two nations are now in bed with each other’s adversaries,” Tol said. Turkey has worked with jihadi groups in Syria and extremists in Libya, while the United States has partnered with a Kurdish-led militia in Syria. Erdoğan views the Kurds—the country’s largest ethnic minority, which spills into Syria, Iraq, and Iran—as a strategic threat. Kurds have actively participated in politics, but a militant faction, the Kurdistan Workers’ Party, or P.K.K., launched a violent insurgency in the nineteen-eighties in response to Turkish repression.

The deepening strategic divide between the U.S. and Turkey is reflected in their split over the status of two men with disparate ties to the two nations. For the United States, the most important figure in the five-year campaign to defeat ISIS was General Mazloum Kobani Abdi, the nom de guerre of the Kurdish leader of the Syrian Democratic Forces. Mazloum lost more than eleven thousand troops, men and women, fighting Islamic State jihadis in Syria, in a partnership with more than seventy nations in a U.S.-led coalition. Trump has spoken to Mazloum twice in recent weeks and praised him repeatedly. Last month, a bipartisan group of senators petitioned the State Department to expedite a visa for Mazloum to visit Washington.

At Trump and Erdoğan’s joint press conference, however, Erdoğan called Mazloum—whose real name is Ferhat Abdi Sahin—a terrorist. “So a person like this should not be welcomed by a country such as the United States,” Erdoğan said. Turkey has invoked an Interpol “red notice” on Mazloum; it requires any member state to arrest a wanted person. Prospects of a visit by Mazloum to the U.S. now seem dim.

Turkey and the United States also disagree on the status of an elderly Turkish cleric named Fetullah Gulen, who has lived in Pennsylvania’s Poconos for two decades. His followers in Turkey formed the global Hizmet (or Service) movement, which operates modern schools in Muslim communities, fosters moderate Islam, and engages in interfaith dialogue with Christians and Jews. This informal movement reportedly has millions of followers, from Kenya to Kazakhstan. Gulen’s supporters initially helped Erdoğan’s A.K.P. Party rise to power, in the nineteen-nineties, and Gulenists gained important positions in government, the security forces, businesses, and the civil service. But Erdoğan grew suspicious of the movement’s political intentions. He blamed Gulen and his supporters for the failed military coup in 2016. He has repeatedly demanded that the United States extradite Gulen for trial on charges of terrorism and treason. The United States, under both the Obama and Trump Administrations, has countered that there is no evidence that the reclusive cleric was involved in the failed plot. At the press conference on Wednesday, Erdoğan again alleged that Gulen had tried to “destroy the constitutional order of Turkey.” He demanded that the Administration “eradicate” Gulen’s presence in the United States.