Sensing that its protracted efforts to purchase an air defense missile system from its U.S. ally were not heading for success, Turkey signed a contract with Russia in April 2017 to acquire the S-400 anti-missile shield.
Opposing the deployment of the Russian system, U.S. officials argued that they would be incompatible with existing NATO systems and expose its advanced fifth-generation F-35 jet to possible Russian subterfuge.
Turkey, however, emphasized that the S-400 would not be integrated into the NATO systems, and therefore had no chance to pose any threat to the alliance or its armaments.
Turkey even asked for setting up of a commission to clarify any technical issues. But the U.S. has, so far, not responded to this proposal.
The U.S. began the process of removing Turkey as a partner on the multinational F-35 program in April after Ankara said it would not back down from its S-400 purchase. Tensions deepened in July when Turkey received its first shipment of the Russian equipment.
Washington has warned it would impose sanctions beyond removing Turkey from the F-35 program.
Ankara said it was the U.S.' initial refusal to sell it Patriot missiles that led it to seek out other sellers, adding that Russia offered it a better deal, including technology transfers.
The S-400 system is seen as one of the most advanced missile systems in the world, capable of tracking multiple targets simultaneously
