Erdogan’s purchase of Russia’s S-400 crosses a NATO red line.

Turkish President Recep Tayyip Erdogan has spent 15 years chipping away at his country’s reputation as a stalwart Western ally. Now Turkey’s deepening military ties with Russia are a breaking point that should prompt Washington to re-evaluate its broader relationship with Ankara.

The latest flash point came this month when then acting Defense Secretary Pat Shanahan told Turkish Defense Minister Hulusi Akar his country would be kicked out of America’s F-35 jet fighter program unless it canceled its purchase of the Russian S-400 antiaircraft system. Moscow says it will deliver the missiles in July, and the U.S. already has stopped F-35 training for Turkish pilots.

This disrupts the F-35 program, which Ankara has invested more than $1 billion in since 2002. The country’s order of 100 jets will need to be backfilled, though Lockheed Martin says that won’t be a problem. More worrisome, Turkey plays an important role in the F-35 global supply chain, as eight Turkish companies have supported production of the jet. Reconfiguration means price hikes and delays, but it’s worth the hassle.

Any $2.5 billion Russian arms deal with a NATO ally would raise eyebrows, but the antiaircraft system is a unique threat for the U.S. and its allies. “The S-400 is a computer. The F-35 is a computer,” Pentagon official Katie Wheelbarger said in March. “You don’t hook your computer to your adversary’s computer and that’s basically what we would be doing.”

The showdown raises bigger questions about Turkey’s place in the world. For decades the country was a model of how a Muslim-majority country with a secular government could cooperate with the West. Now Mr. Erdogan is operating in a more ambiguous space: becoming more authoritarian while balancing the U.S. and Europe against Iran and Russia.

Much of this shift is driven less by strategic vision than Mr. Erdogan’s desire to remain in power. Unsettled by a 2016 coup attempt, the president sped up his curtailment of civil liberties. The U.S. often has to work with unsavory authoritarians, but Mr. Erdogan’s abandonment of Western values has coincided with more strategic confrontations. Turkey has undermined U.S. interests in Syria while complicating America’s campaign to isolate Iran. The S-400 purchase delights Vladimir Putin as he pulls Turkey from the West.

Washington still benefits from its alliance with Ankara—not least from using the Incirlik Air Base—and shouldn’t give up on repairing the relationship. Turkey, looking at Iran and Russia, also understands the costs of alienating the West with an already fragile economy.

The F-35 withdrawal is a good step, but the U.S. has other means to show Turkey that bad behavior has real consequences. In 2017 Congress passed the Countering America’s Adversaries Through Sanctions Act (Caatsa), which imposes sanctions on countries that buy arms from Mr. Putin’s government. The White House shouldn’t interfere in the imposition of Congressional Caatsa sanctions.

U.S. officials will also have to consider alternatives to Incirlik. Ousting Turkey from NATO would be a last resort, but the tragedy of Mr. Erdogan’s rule is that it’s now thinkable.

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