(Bloomberg Opinion) -- If the North Atlantic Treaty Organization were any ordinary club, it wouldn’t hesitate to kick out one of its members: Turkey. The country has long been difficult; in 1974 it even fought against another member, Greece. But under President Recep Tayyip Erdogan, Turkey has increasingly seemed less frenemy than outright foe. And yet NATO can’t simply push it out. That’s because Turkey might go nuclear, and not in a metaphorical sense.

The case for ejecting Turkey is certainly compelling. This week, as NATO leaders tried gamely to appear united after recent spats and divisions, Erdogan blithely went the other way. He threatened to block a plan to better protect the Polish and Baltic allies from Russia, NATO’s most obvious enemy, because he wanted NATO in return to designate the Kurdish YPG militia as a terrorist group. The YPG was until recently an American ally in the fight against Islamic State in Syria, so that wasn’t going to happen.

In the end, Erdogan relented. Yet he seems to consider the Kremlin a partner more than a menace. He recently collaborated with Russian President Vladimir Putin in invading northern Syria, and has purchased a Russian air-defense system that could potentially spy on — or even sabotage — NATO’s other equipment, including U.S.-made fighter planes.

And those are just the military aspects of a relationship that has become hostile all around. Turkey is also drilling for oil and gas in the Mediterranean, ignoring the objections of the European Union, which has dibs on the same waters. And it is becoming less democratic by the day. Since an attempted coup in 2016, Erdogan has turned unabashedly authoritarian, cracking down on the press, the courts and civil society.

Despite all these concerns, NATO must learn to live with Turkey, owing to two cold realities.

First, Erdogan could actually make good on his threat to send migrants into Europe in numbers that exceed the populations of some EU countries. Turkey accommodates 3.7 million Syrian refugees. It could nudge them to cross to the Greek islands, as about 200,000 people did every month at the height of Europe’s migrant crisis in 2015.

One reason those crossings all but stopped the next year was that the two sides reached a deal: Turkey agreed to take back refugees that made it to Greece in return for lots of money and other help from the EU. Quitting that deal would hurt Turkey. But faced with the humiliation of being cast out of NATO, Erdogan might decide that it’s worth it. The EU, meanwhile, has failed to reform its migrant system since 2015; it would enter another crisis.

An even greater peril is that Erdogan would respond to expulsion by building his own nuclear weapons. He definitely wants them. The West says that Turkey “can’t have them,” he ranted in October. “This I cannot accept.”

Nuclear proliferation is one of the world’s most dire problems as it is. One arms-control treaty between the U.S. and Russia (on “Intermediate-Range Nuclear Forces”) recently fell apart; the remaining one (called “New START”) seems destined to lapse in 2021. And as the Nuclear Non-Proliferation Treaty turns 50 next March, it looks increasingly toothless. From North Korea to Iran, the wrong people have, or are capable of getting, the wrong weapons.

A Turkish bomb in the Middle East would therefore be a disaster. It would almost certainly lead to an arms race, as not only Iran but Saudi Arabia joined Israel in going nuclear. Superimposed on a map of conflicts that makes Europe in 1914 look simple, this is a recipe for Armageddon.

NATO can’t let that happen. Nor, of course, can it simply shrug off the political and military implications of a member state that might turn on its allies. It must draw up plans for new and scary scenarios. And it should move the nuclear weapons the U.S. has currently stationed in Turkey, at its base in Incirlik, to Europe. This won’t solve the bigger problem. But it’s a necessary first step beyond denial.

To contact the author of this story: Andreas Kluth at akluth1@bloomberg.net

To contact the editor responsible for this story: Timothy Lavin at tlavin1@bloomberg.net

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Andreas Kluth is a member of Bloomberg's editorial board. He was previously editor in chief of Handelsblatt Global and a writer for the Economist.

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