US-Turkey relations soured after Turkey purchased the Russian S-400 missile defense system. The first deliveries were made on Friday. The US warned Turkey not to buy the equipment from Russia. The USA and Turkey are both allies in NATO – a defense organization created to counter a military threat from Russia. The USA has the biggest military in NATO. Turkey has recently established closer ties with Russia. This has worried the USA.

The U.S. warned Turkey of economic sanctions if it installed the S-400 system. It also said Turkey would not be part of the USA's F-35 fighter jet programme. U.S. officials say the Russian system is incompatible with NATO systems and would risk the security of F-35 fighter jets. Turkey will not integrate the S-400 with NATO systems and says there is no threat to NATO. Turkey said buying the S-400 did not mean it is seeking "alternatives" in relations with the West.