The handwriting was on the wall and now the ongoing flap between the United States and Turkey over the U.S threatening to impose sanctions on Ankara, for buying a Russian missile system, is reaching a tipping point.

Turkish President Recep Tayyip Erdogan threatened to close two strategic U.S. airbases at Incirlik and Kurecik over these sanctions. 

“If necessary, we’ll close Incirlik and also Kurecik,” Erdogan said on Sunday. “If the threat of sanctions is implemented against us, we’ll respond to them in the framework of reciprocity.”

Turkish Foreign Minister Mevlut Cavusoglu also brought up U.S. base closure last week when the threat of sanctions was brought up by Washington. He stated that the base closure would “be put on the table.”

Incirlik has been a U.S. airbase since 1955. It has been a strategic spot for the United States to project power in the region. It was a key base during the Gulf War in 1991, the Global War on Terror, the invasion of Iraq and the recent campaign against the Islamic State in the Middle East. It is also home to 50 of the B-61 nuclear gravity bombs. Kurecik is a key NATO radar base  

Members of Congress have been pushing President Trump to impose sanctions on Turkey for buying the Russian S-400 missile system. They argue that this system is incompatible with membership in NATO and is a threat to the new F-35 fighter jets, because of fears that the Russians will learn valuable information about the F-35 if the Turks use both systems at the same time. 

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Because of the Turkish purchase, the United States kicked Turkey out of the F-35 program, to which it had been a partner.

This move by Erdogan is seen as purely political and a signal that Turkey will not bow to either the U.S. or NATO in matters of their defense.

Congress is proposing using the U.S. law known as the Countering America’s Adversaries Through Sanctions Act (CAATSA), which states that buying certain military hardware from Russia must be sanctioned. Thus far, President Trump hasn’t signed off on any sanctions.

At the heart of the matter are the S-400 Triumph anti-aircraft missiles. They are known as the SA-21 Growler in NATO. The Russians claim that the S-400s are superior to anything that the United States or the French use. 

The S-400s are mobile and include the control hub and several missile launching elements, each of which is comprised of up to 12 launchers. It has a range of 400 kilometers (250 miles) and can hit targets at an altitude of 27 kilometers. Their cost is about half of that of the U.S.-built Patriot 2. 

Russia has deployed them to the Crimea and Syria. It has sold them to China, India and now to the Turks. The S-400 is designed to destroy fighter jets, cruise missiles, ballistic missiles and drones. It has yet to be used in combat, however. In Syria, although deployed, they have yet to be fired, as the Russians have been holding them back — even after the U.S. fired Tomahawk cruise missiles at targets in Syria.

Another sticking point between Ankara and Washington is the invasion by Turkish troops into northeastern Syria. After the United States pulled out of some of its bases, the Turks crossed the border in force, attacking the Kurdish troops that had partnered with the U.S. in fighting the Islamic State. 

Ankara considers the Syrian Kurdish YPG militia, which was supported by U.S. Special Operations Forces, to be a terrorist group with links to Kurdish rebels who have been fighting a 35-year insurgency against Turkey.

And if relations can’t get any worse, the Turkish government is upset with Washington’s decision to declare the mass killing of Armenians in 1915 as genocide. Turkey claims that the number 1.5 million deaths, as stated in the U.S. resolution, is inflated and responded that many Turks also died, blaming the violence on World War I. President Trump has yet to sign that resolution either.