The controversy surrounding Turkey’s purchase of the Russian S-400 missile defense system has been brewing for months and may soon be coming to a head. The strategic and technical aspects of the move, as well as its wider regional geopolitical context, have been amply documented. But what would the longer-term military and political implications be of a deployment of Russian S-400s?

Turkey has used two arguments to justify buying a missile system from Russia rather than the United States: First, it says that Washington has been unwilling to transfer technology to Turkey—though Moscow has denied that its sale of S-400s would include a transfer of technology. And second, Turkey does not want its military procurement to be tied exclusively to its Western partners.

Then there is the question of the F-35 stealth fighter, which the United States is supposed to supply to Turkey. For the Americans as well as the many other NATO countries that have ordered the aircraft, maintaining the integrity of the F-35 is vital since it will be their standard aircraft for decades. Therefore, any access by Russia to its electronic “signature” on S-400 radars is simply a no-go. Turkish counterarguments that Turkey would operate the S-400 system without Russian technical assistance (though Moscow has confirmed the system would come with Russian personnel), or would use them independently of its air defense architecture, are unconvincing.

Several commentators have speculated that Turkey could cancel the S-400 deal after the country’s March 31 municipal elections. However, that would mean Ankara’s heavily damaging its budding relationship with Moscow. Conversely, if the S-400 system is delivered and activated, sanctions could be imposed by Washington—from delaying deliveries of the F-35 to excluding Turkey from its production chain. Beyond these uncertainties, a deployment would entail deep military and political repercussions.

Assume that S-400 batteries, manned by Turkish Air Force personnel and seconded by Russian “trainers,” are deployed in late 2019 at two non-NATO bases—for example one near Ankara, the other in Eastern Anatolia. In reaction to this, Turkey’s political leadership would have a feeling of newly-found “strategic independence,” an argument also used to justify the purchase. But in reality it would have entered a stage of “strategic dependence” on Russia. Turkey would be a prisoner of the Russian presence on its soil—with hardware, software, and personnel. This dependence would only deepen if Moscow were to provide Turkey with Russian Su-57 stealth fighters should Washington cancel the F-35 deliveries.

Some of the hypothetical questions raised by such developments would be the following: How could Turkey act in solidarity with NATO in case of an eventual crisis with Russia in eastern Ukraine or the Baltic while hosting Russian missiles in two air force bases? How could it keep critical U.S. and NATO assets deployed at three major Turkish bases—Incirlik, Konya, and Malatya, among others facilities—while having Russian assets deployed at other bases?

Iran possesses one of the largest missile inventories in the Middle East. Two of its most powerful missiles, the Shahab and the Sejjil, are capable of reaching all of Greece, Israel, and Turkey, most of Bulgaria and Romania, and all the Western military bases in Crete, Cyprus, Djibouti, and the Gulf. In case of an acute crisis with Tehran, would Turkey be able to fulfill its commitments to NATO while hosting Russian S-400 missiles and partnering with Russia and Iran in Syria? If suddenly drawn into the crisis, would Turkey call NATO for help or use its Russian missiles? Could Moscow retain “veto power” over the use of the S-400s in such circumstances?

Going further, would the United States ultimately want to reorganize its presence at the Incirlik air force base, where several dozen nuclear warheads are prepositioned? Would a restructuring of NATO activities affect its forward operating base in Konya, where its E-3A AWACS help in the operations of the anti-Islamic State coalition? More generally, would NATO countries have to rethink the role of their forward bases in the Mediterranean and the Middle East—both in European Union (EU) countries such as Cyprus, Greece, and Italy, and in Turkey and Arab countries such as the United Arab Emirates, Jordan, and Qatar—in order to take into account the uncertainties stemming from the S-400 deployment in Turkey?

Ultimately, would NATO’s trust in Turkey’s military command—already dented by the deep political purge since the coup attempt of July 2016—vanish as the country is seen to be strategically aligning with Russia? At the very least, NATO operational procedures would have to be scrutinized. Some analysts, such as those at the Center for American Progress, argue that NATO should explore steps “to extricate Turkey from certain activities aimed at countering Russia, such as the Very High Readiness Joint Task Force and certain intelligence sharing arrangements.” More generally, would NATO continue to include Turkish soldiers in its operations to counter Russian military activities around Europe? In other words, would Turkey’s participation in NATO activities continue uninterrupted?

If it happens at all, deployment of the Russian missiles at the heart of Turkey’s air defense network will have repercussions beyond the military sphere. The West’s trust in Turkey has already been cracked, not only on missile procurement but also on other important subjects. These include differences over the anti-Islamic State coalition, gas exploration in the eastern Mediterranean, the transit of Russian gas through Turkey, Russian political interference in the domestic politics of European countries, a Turkish narrative calling Western leaders “enemies of Islam,” and differences with regard to societal choices in Turkey incompatible with EU and Western standards.

If the S-400s are eventually deployed, Turkey’s drift away from the West would become real. The international consequences are potentially momentous. Beyond the changes in NATO policies, it would represent a big gain for Moscow’s muscle-flexing around Europe. It would also confirm that Turkey’s long-lasting, strategic European and Western anchor can no longer be taken for granted.

The eastern Mediterranean and Middle East are undergoing tectonic changes. Consecutive U.S. presidents have demonstrated a lower proclivity for the United States to remain the regions’ peacekeeper. Russia and Iran, in turn, have shown a strong desire to become perennial military actors there and beyond. Until now, Turkey has been part of the post-World War II, post-Cold War defense network of the North Atlantic alliance in the region. However, its forthcoming decisions on missile defense will constitute a test for the Trump administration’s foreign policy, for the foreign policy of the EU with regard to its neighborhood, and not least for Middle Eastern countries as well. This is no small development.