RELAX, WHAT COULD GO WRONG? What’s Next in the Terrifying, Unraveling Russo-Turkish Crisis.

Mr. Putin and Mr. Erdoğan are charismatic strongmen with deep nationalist credentials. They have successfully employed faith mixed with chauvinism to handsome political effect, including nostalgia for lost imperial glory that rankles and scares their neighbors. They are popular with many citizens, who credit them with big economic advances among average people. Their political foes have fled the country in fear, while dissenting journalists and activists get arrested or killed in “mysterious,” never-solved crimes. Above all, they have used quasi-democratic ends to establish very un-democratic regimes, personally profiting in the process. Neither man has any history of backing down in the hour of crisis.

If all this sounds alarming, it should. . . .

In Mr. Erdoğan, Mr. Putin has encountered a foe whose congenital response to the Kremlin strongman’s usual foreign policy playbook of tantrums and threats will be pushback rather than backing down. This matters because Turkey is a key member of NATO and it possesses a large and competent military—though it lacks the several thousand nuclear weapons Mr. Putin controls. Alarming signs are not difficult to detect. Demands in Moscow that Turkey return Hagia Sophia to the Orthodox Church, which fell to the Ottomans in 1453 when Constantinople—now Istanbul—at last was taken by the Turks after centuries of effort, are sure to inflame passions among history-minded Turks of an Islamist bent like Mr. Erdoğan.

Falling back on militant faith and historical grievance in a crisis is seldom an encouraging sign, and NATO needs to make it abundantly clear to Ankara that Article 5, the Alliance’s collective defense clause, does not apply if Turkey goads Russia into an avoidable war. That said, Moscow must understand that it cannot bully a NATO member without consequences either.

Ideally, both Putin and Erdogan would lose, but unfortunately, a lot of other people would lose along with them.