If you think Turkey’s relationship with the United States is currently very tumultuous, you haven’t seen anything yet.

The two allies have had bouts of hysteria on and off since 2015, mostly to do with Ankara resenting U.S. support for Syrian Kurds and Washington increasingly finding its longtime ally an unreliable partner in the Middle East. Turks have never forgiven Washington for what they believe was tacit American support for the failed 2016 coup, and Americans have not fully come to terms with Turkish President Recep Tayyip Erdogan’s project to turn his country into an illiberal Middle Eastern regime. So dysfunctional has the relationship become that the Trump administration ended up imposing sanctions on two senior Turkish government officials last year in order to secure the release of an American pastor jailed in Turkey.

But what we have seen so far may pale in comparison with what is to come.

Turkey’s decision to purchase the Russian made S-400 defense missile system is possibly the mother of all crises in the decades-long alliance. For the United States, the deployment of advanced Russian military hardware by a major NATO ally threatens NATO secrets, sets a precedent that could harm lucrative U.S. arms sales and makes Turkey ever more dependent on Russia. Washington has threatened to slap Turkey with sanctions if it takes the delivery — to include a ban on the sale of next-generation F-35 jets.

Turkey says the S-400 deal with Russia is already done but that it could also buy U.S.-made Patriot missiles as well, even though the country is going through a massive economic downturn and the purchase of two big systems would be a huge burden. Washington insists that Turkey must pick just one.

But the dispute is about much more than which expensive toy Turkey buys. The Russians are trying to use cracks within NATO to destroy whatever is left of the “Western alliance.” Turkey is vulnerable because Erdogan finds it hard to move in Syria without Russian consent. Unlike Americans and Europeans, Russian President Vladimir Putin doesn’t care how Erdogan runs his country — and Erdogan appreciates that.

The outcome of the S-400 imbroglio will define Turkey’s place in the West, or usher in its formal exit from the transatlantic community. Losing a significant NATO member to Russia would certainly weaken the alliance and lead to the proliferation of Russian systems.

Still, it’s not clear how much President Trump cares about all of this. His administration has been too late to start lobbying against the S-400s, putting Erdogan in an awkward position with the Russians only months before the July delivery. The bromance between Erdogan and Trump hasn’t helped much. In a recent phone call, Erdogan offered to purchase both the Russian and American hardware and counseled the U.S. president to convince Congress on that. That’s exactly what Congress is unwilling to do, knowing that the arrival of Russian hardware would entice others to follow — and that Trump cannot be fully trusted with this decision.

The next four months will probably mark an escalation. There is already manic maneuvering by everyone except the Russians. Putin is the ultimate mafia boss, and from Erdogan’s viewpoint, it is probably harder to say no to him than to Trump. Moreover, the Russians are offering Erdogan not just a weapons system but in some sense a permanent protection for his seat, as they have to their other allies, like Venezuela’s Nicolás Maduro.

Ankara has put itself in such a vulnerable position with Moscow that the moment it cancels the order, Putin could start an offensive against the last regime holdout in Syria, Idlib, driving some 3 million people toward the Turkish border.

The Americans, on the other hand, are offering not just Patriots but also a seat in what is still called “the West," an alliance that lacks self-confidence and direction but is still the best club in town. Even if Erdogan no longer believes in the concept, he knows that drifting from the West would be a big blow to Turkey. It would bring further deterioration of Turkish democracy and lead to the loss of investor confidence in its economy, and the Turkish president certainly cares about the latter.

Erdogan also knows he can negotiate more. American red lines are usually drawn in sand, and he will try to persuade Washington to accept a compromise — perhaps Turkey will buy the S-400 but keep it in a warehouse or send it to a third party. Along the way, he would ask for the head of the Syrian Kurds on a gold platter, and it is not hard to imagine Washington giving it to him.

But even if there is a last-minute deal, this will not fix Turkey’s problems. As much as a strongman as Erdogan appears, the S-400 controversy actually demonstrates the weak position he has put Turkey in. His effort to demonstrate that Turkey has choices and is not an American vassal has so alienated Turkey’s Western partners and weakened its economy that the country now risks becoming a Russian vassal. The Western alliance has its discontents, but over the years it has helped make Turkey rich and strong. Putin’s Russia has no such plans for Turkey.

Whatever weapons the Turkish government buys will not change that.

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