The case for evicting Turkey from NATO got stronger this week.

First, the United States announced the backing of a border security force drawn mainly from the People's Protection Units (YPG) in Rojava, the quasi-independent Kurdish region in northeastern Syria along the Turkish border. Then Turkey's President Recep Tayyip Erdogan says he will "strangle" that American-backed force "before it's even born." Russia, Iran and Syria's Assad regime are standing with Erdogan.

The YPG, along with the multiethnic Syrian Democratic Forces which the YPG dominates, are the only armed groups indigenous to Syria that are willing and able to take on ISIS and win, and they're the only significant armed faction in Syria's dizzying civil war that isn't ideologically hostile to the West. In October of last year, they finally liberated Raqqa, the "capital" of the ISIS "caliphate," while the Russian and Syrian militaries were busy pounding rebels instead in the west.

The Turks would rather have the Assad regime—and by extension Russia, Iran and Hezbollah—rule over the Syrian Kurds, whom they consider terrorists. The United States is "building an army of terror" along the southern border, Erdogan says. "Either you take off your flags on those terrorist organisations, or we will have to hand those flags over to you, Don't force us to bury in the ground those who are with terrorists...Our operations will continue until not a single terrorist remains along our borders, let alone 30,000 of them."

This is not how a NATO ally behaves. It's how an enemy state behaves.

This is not how a NATO ally behaves. It's how an enemy state behaves. There is truly no getting around this. We can argue all we want—and I have—that keeping Turkey in NATO is better than kicking Turkey out of NATO because it's better to deal with a troublesome country inside an ostensibly friendly framework than outside one.

There are limits, though, even if those limits aren't clearly defined. A direct Turkish attack against the United States would clearly be over the line whether a line is defined or not, as would a direct attack against another NATO member state. Attacking a non-NATO ally is more ambiguous, especially when the non-NATO ally in question isn't even a state. (It's not like Turkey is threatening to attack Israel, Japan or Morocco.)

None of this could have been foreseen when NATO was founded in 1949 or when Turkey was admitted in 1952. NATO was founded as a united Western front against the Soviet Union, which occupied or indirectly controlled half of Europe, including a third of Germany. Iran's Islamic Republic, the Syrian Baath Party regime, armed Kurdish separatist movements, ISIS—none of these even existed then, and only the Kurdish movements could even have been imagined.

The world has dramatically changed, as has NATO. In 1952, Turkey was a crucial member in good standing while Estonia was part of the Soviet Union. In 2018, Estonia is a member in good standing while Turkey is behaving as a belligerent. No one should be surprised that alliances have shifted after seven decades. Alliances always shift over time. Enemies become friends and vice versa. Not even Britain has been a constant friend of the United States, and not even Russia has been a constant enemy.

Ankara allowed ISIS to transport men and materials across the Turkey-Syria border before later turning against it.

Changes like these happen slowly, and the West is having a hard time processing the fact that Turkey is increasingly hostile, though it has been for some time now. It started when Ankara denied the use of its territory, including Incirlik Air Base, during the war against Saddam Hussein, mostly because Turkey didn't want Iraqi Kurdistan to become an economic and military powerhouse. Later, Erdogan helped Iran transfer weapons to Hezbollah in Lebanon and implicitly sided with ISIS in Syria because he didn't want an independent Kurdish region to rise up in Syria as it had in Iraq. More recently, he has taken American citizens hostage and purchased a missile system from the Kremlin. And how he's threatening to destroy the only competent Western-friendly militia in all of Syria.

Last August, as Erdogan visited his "dear friend" Vladimir Putin in Moscow, NATO issued a telling statement. "Turkey is a valued ally, making substantial contributions to NATO's joint efforts... Turkey's NATO membership is not in question."

Stop right there. Of course Turkey's NATO membership is in question. Otherwise, why bother denying it? NATO isn't denying that the United Kingdom or Canada doesn't belong in NATO any longer. NATO is only denying that Turkey's membership is in question, which is another way of saying it is. Anyway, you can type "Turkey out of NATO" into Google and spend a year wading through the results.

If Turkey were not already in NATO, it would never be admitted.

The statement continues: "Our Alliance is committed to collective defence and founded on the principles of democracy, individual liberty, human rights and the rule of law." Indeed, the alliance was founded on all of those principles, none of which increasingly authoritarian Turkey adheres to any longer.

If Turkey were not in NATO, it would not be admitted. It's grandfathered in at this point.

It's much easier to say no to an aspiring member that doesn't belong than to evict a longstanding member who no longer belongs, especially when there's no clear criteria for banishment. It's about time, then, for NATO to have a serious discussion about what the criteria for banishment is. That alone might improve Turkey's behavior. If it doesn't, we'll have other options.