Erdogan shakes hands with Russian President Vladimir Putin in Ankara, September 2017.

On September 12, 2017, Turkey signed and made a deposit toward a $2.5 billion agreement to purchase Russia’s S-400 anti-aircraft missile system. Russia will sell Turkey two batteries and transfer the technology for Ankara to produce two of its own, while Turkey will supply the batteries’ target-identification software. As Pentagon officials have noted, the S-400 is not interoperable with NATO’s air defense system, causing some experts to speculate about Turkey’s relationship with the alliance.

The S-400, a state-of-the-art missile defense system, is overkill in defending against non-state armed groups such as the Islamic State (ISIS) or the Kurds. It also

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