THE COLLAPSE of the Soviet Union touched off a string of border wars in Eurasia, the bloodiest of which centered on Nagorno-Karabakh, a mountainous, Armenian-populated enclave inside Azerbaijan. After Armenian forces drove the Azeris out of the region and captured adjacent territories, a 1994 cease-fire created one of the “frozen conflicts” that haunt the post-Soviet space. In the past week, after years of quiet, the fighting has erupted again, with the explosive potential to renew carnage that killed tens of thousands, disrupt European energy supplies and draw in neighboring powers.

As always, there are conflicting accounts of which side started the shooting. But what seems clear is that Azerbaijan’s autocratic ruler, Ilham Aliyev, has launched an offensive to regain the territories his country lost in the 1990s — and that he is doing so with the direct support of Turkey. It’s a reckless gambit that reflects both the shrinking influence of the United States under President Trump and the mounting ambitions of his sometime-friend Recep Tayyip Erdogan, Turkey’s autocratic ruler.

Turkey was once a reliable NATO ally that hewed to the West’s strategic priorities. But in the past several years Mr. Erdogan has increasingly sought to make his nation into a geopolitical power in its own right, even as he has dismantled its democracy. In the past few months Ankara has supplied materiel and fighters to one side of Libya’s civil war; risked war with Greece over disputed waters in the eastern Mediterranean; and intervened to block an attempt by the Syrian government to recapture the northern province of Idlib.

Now Mr. Erdogan is offering full-throated supported to the Azeri offensive, while demanding that Armenia withdraw from Nagorno-Karabakh. According to France and Russia, as well as independent reports by news organizations, Turkey has transported hundreds of Syrian mercenaries to the battlefield, and Armenian Prime Minister Nikol Pashinyantold The Post’s David Ignatiusthat a Turkish F-16 shot down an Armenian warplane on Tuesday. Turkey’s denials are unconvincing.

Mr. Erdogan’s initiative has placed him at odds with Russia, which has a military base in Armenia and is pledged to defend it from aggression. He and Mr. Aliyev have so far ignored calls for a cease-fire from Russia, France and the United States, including a joint statement from their presidents on Thursday. Those three countries have sponsored Armenia-Azerbaijan peace talks for years, but Mr. Aliyev said Tuesday that “any negotiating process is out of the question” because of what he said are unacceptable demands by Armenia.

It’s not clear how far Russia will go to stop the Turkish-Azeri offensive; Mr. Pashinyan was democratically elected in 2018 after a popular uprising, and so is no favorite of Vladimir Putin. But it is in the U.S. interest to stop the fighting and restart negotiations. That will require reining in Mr. Aliyev and Mr. Erdogan and their exaggerated ambitions. Mr. Trump, who has been an open admirer of the Turkish strongman, ought to tell him to stand down.

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