A Turkish purchase of a Russian-made missile-defense system threatens to deepen the divide between Ankara and Washington.

On July 12, Russia began delivering components of its advanced S-400 missile defense system. That triggered a defensive reaction in the U.S., which worries that the Russian system could collect intelligence on U.S. fighter jets like the F-35, which Turkey was slated to buy.

More profoundly, it sparked concerns about Turkey’s tilt toward Moscow and whether the S-400 purchase advances Russian President Vladimir Putin’s enduring goal of weakening the Western alliance.

Putin’s plan may indeed be working. “Clearly a winner in all this is Putin,” said Ross Wilson, who served as U.S. ambassador to Turkey from 2005-2008. Wilson, who now is board chair at Global Minnesota, added that, “It’s been Russia’s mission going back to Soviet times to divide the alliance, and Putin is succeeding.”

A previous Soviet-era rift over Turkey’s role in Cyprus was not easily solved despite both nations seeking a solution. Now, deep divisions spanning multiple administrations over multiple issues make the split seem more intractable.

Among these simmering issues is Turkey’s frustration over what it believes is the balky pace of U.S. arms sales, as well as Turkish President Recep Tayyip Erdogan’s perception of U.S. insensitivity over a 2016 coup attempt (which Erdogan believes was plotted in part by a Turkish cleric, Fethullah Gulen, who now resides in Pennsylvania). There were also divergent American and Turkish goals in Iraq and Syria, where the U.S. worked with Kurdish militias that Turkey resolutely opposes.

Overall, these disputes “have really created deep resentment in both capitals,” so most of the decisions made in Washington and Ankara “are now marinated in anger,” Soner Cagaptay, director of the Turkish Research Program at the Washington Institute of Near East Policy, told an editorial writer.

“So an eruption may not be so easy to mend this time,” said Cagaptay, who added that, “I think it’s a pretty severe set of developments.”

Things may get more severe if (or more likely, when) sanctions are triggered by Turkey’s purchase of the Russian system. The Trump administration has some leeway in their severity, but because Erdogan controls Turkish media any sanctions will be framed as a U.S. attack on its sovereignty.

Still, the administration rightly demanded that Ankara make a choice between purchasing the Russian missile-defense system and continuing its partial manufacturing and planned purchase of America’s latest fighter jets.

Erdogan, as he has so often in his increasingly repressive rule, chose wrongly, and the bilateral relationship will soon suffer new strains.

And the alliance matters. “Turkey is an important ally; it borders Iran, Iraq, Syria, Russia across the Black Sea, ISIS-held territory across the south, so whatever America’s policies are it’s much easier to execute them with Turkey on board,” Cagaptay said.

“I think what’s happened is deeply damaging to American interests, it’s deeply damaging to U.S.-Turkish relations,” Wilson said. “It is a huge disaster, and I think it really threatens to upend a relationship that’s always been difficult but has been a cornerstone of American policy going back to the 1950s in that part of the world.”

Neither side can afford to relinquish that bond. Diplomacy won’t be easy, but it’s essential for Turkey, the U.S. and the West.