The US refused to train Turkish pilots because of the Russian S-400

The US will cease to accept Turkish pilots on program training skills piloting fighter jets-bombers F-35.
This measure is due to Ankara’s decision to purchase S-400 anti-aircraft missile systems from Russia. It will affect only those pilots who were to begin training at the Luke airbase in Arizona.
At the moment, four pilots from Turkey are trained at the American airbase, two more work as instructors, 20 citizens of this country are responsible for the operation of military aircraft.
On May 5, the US offered Turkey a few options in exchange for refusal from the purchase of the Russian S-400. It was noted that Washington gave Ankara “understand the consequences” of its choice, after which the decision remained with the authorities of the country.
On May 29, it was reported that the US government was “seriously considering” the possibility of suspending the training of Turkish military pilots in the US, but the official decision was not announced.
The agreement on the supply of four S-400 divisions to Ankara was reached in September 2017. The cost of the contract is $ 2.5 billion, but half of this amount will be covered by the Russian loan.