What happened?

The US authorities have begun the process to exclude Turkey from the F-35 program. The move comes after several rounds of negotiations. Turkey had called for reconsideration, but the US chose the harsher response. Donald Trump on Thursday said that imposing sanctions on Turkey for its purchase of the Russian made S-400 Triumf missile defence system is still under consideration in the US administration. This comes after the US ended Turkey's involvement in the development of the F-35 fighter jet. 

By March 2020, the withdrawal of Turkey's industrial participation in the development of the fighter jet will be completed.

What is the background?

The tensions between the two countries have been on for several years. Sharing a long history of alliance from the Cold War era where they found a common enemy in the Soviets. The interests of the two nations diverged in the case of Syria. The US support to Kurd rebels made Ankara dubious of its real interests. 

Though a NATO partner, Ankara has grown sceptical of how important Turkey's security to Washington is. The watershed moment could be the 2016 coup attempt on Erdogan. While the cleric who ordered the coup was located in the American soil and a lukewarm response by the US annoyed Turkey, Vladimir Putin was one of the first international leaders to phone Erdogan. 

The S-400s are the best anti-aircraft defence system in the world, quintessential for Erdogan to defend his palace against the NATO jets flown by rogue pilots that are proving to be a security risk. Hence the alliance seems to be also out of necessity.

What does it mean?

While Turkey is diverging from its earlier position in the world stage as a NATO partner, as a staunch ally of the US against the Russians in this radical pivot move, the one that would benefit the most would be Russia. Russia has been looking to increase its global footprint somewhat slyly by signing agreements and deals with several nations facing sanctions from the US. 
About Turkey, the case is no different. An offer for Su-35 fighter jets as a replacement for the F-35s is already on the table from the Russian Rostec corporation. 

With the Congress calling for enacting sanctions on Turkey through CAATSA, the US is dismayed with the developments. While the US sanctions are undesirable for the nations, it is also becoming a tool for Russia to find common ground to engage more with the affected countries.