Turkish-American ties have been seriously strained over the past few years since the US started arming Syrian-based Kurdish fighters that Ankara considers to be terrorists and the Mideast country's officials strongly implied that Washington had a hand in the failed summer 2016 coup attempt against President Erdogan.
It's little wonder then that Turkey began to reorient itself eastward towards Russia and plans to purchase the S-400s, seeing as how this air defense system could neutralize any prospective threat emanating either from the American Air Force or possibly even an American-backed Kurdish one that might one day take shape in northeastern Syria.
Russia's "military diplomacy" seeks to maintain the balance of power everywhere in the world in order to facilitate diplomatic solutions to seemingly intractable conflicts, which in this context could see Turkey improving its defenses to the point of strengthening its negotiating hand with the US over the coup and Kurdish issues that lay at the heart of their "security dilemma". Moscow also knows that the sale of high-level equipment such as the S-400s precedes the establishment of long-term military-to-military partnerships that could facilitate the recipient's embrace of multipolarity as it seeks to step away from the US' unipolar-controlled sphere of influence.
Such transitions take time for logistical and other reasons, but the process might accelerate in the Turkish case after the US threatened to exclude its nominal NATO partner from the F-35 program, which in turn prompted unnamed Turkish defense officials to reportedly claim that their country can just turn to Russia for replacements instead. In the event that Turkey's purchase of the S-400s leads to the US carrying through on its F-35 threats and Ankara reactively reaches out to Russia for Sukhois or other warplanes, then the intra-NATO crisis would enter a qualitatively new and much more intense phase.
It's not to suggest that Russia's "military diplomacy" was aimed at deviously advancing this "master plan" all along, but just to point out the role that Moscow played in this chain of events that developed outside of its control. Had it not been for the US' support of Syrian-based Kurdish militants and the shadowy role that it probably played in the failed 2016 coup attempt, then Turkey's trust in its decades-long partner wouldn't have deteriorated to the point where Ankara felt compelled to reach out to Washington's rivals in Moscow in order to ensure its national security needs, something that Russia was eager to assist it with.
Turkey is rapidly developing its mutually beneficial strategic partnership with Russia, which serves the grand strategic interests of both multipolar Great Powers. This new Russian-Turkish relationship has greatly stabilized the military situation in Syria and holds the promise of improving the prospects of a political solution to the long-running conflict there, even if it's still somewhat imperfect and some kinks remain to be worked out. Nevertheless, Russia obtains a reliable partner with enormous commercial market potential while Turkey receives reliable energy supplies from a country with impressive military-technology capabilities that it's more than willing to export.
As for the US, it doubly loses because one of its main geostrategic rivals has successfully "poached" one of Washington's top Mideast allies as a result of America's reckless regional policies that got it into this mess in the first place. The US and Turkey will still try to retain some degree of pragmatic relations on issues of shared interest, but the partnership will never be the same again unless President Erdogan and the ruling AKP are deposed of and a vehemently pro-American replacement rises to power, which appears unlikely for the time being but nevertheless can't be ruled out.
Going forward, the odds are that Turkey won't be formally removed from NATO because no such mechanism exists but will instead probably be de-facto isolated from most of the bloc's working activities, especially those related to intelligence sharing and joint military exercises. The US can't take the risk of unraveling the military alliance on which so much of its European influence depends, no matter how divided and ineffective it is in practice, so indefinitely putting Turkey in "time-out" is the most realistic option available to it for mitigating the strategic fallout of Ankara switching sides as a result of Moscow's "military diplomacy".
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