Sorry sir, Looks like we all had very different understanding of Issue with Turkey and US and reasons why Turkey went for S-400 ..
Follow the below tweet thread may be one perspective for the change
S-400s, a story: Everything started when Turkey downed a Russian jet, Su-24 in Nov 2015 for violating its airspace. Russians were repeatedly bombarding Turkman-led rebel groups by using Turkish airspace. Ankara issued several warnings. Then downed a jet. What was US reaction?
US officials, on the record, initially supported Turkey’s arguments. But had different talking points while briefing reporters on background. US defense officials undermined Turkey’s claim and said Russian jet was in Syrian airspace when it was hit. So much for NATO solidarity
What else US thought of Turkey’s downing the Russian jet? They thought it was an overreaction to “a minor violation of its airspace”
What about NATO? Oh, they were generous. They sent an air defence package against Russian incursions following the jet incident, including AWACS. But NATO diplomats WORRIED Ankara was too aggressive to defend its airspace against the repeated violations
Next, Germany/Luxembourg declared that Turkey cannot count on NATO support if there is a conflict with Russia in Syria. “NATO cannot allow itself to be pulled into a military escalation with Russia as a result of tensions between the two” Briefly: You cannot invoke article 4
Btw, US and Germany earlier in October 2015, a month before the downing of the jet, pulled their missile defense systems from Turkey. Not only US and NATO didn’t want to support Turkey against Russia, they also didn’t want to keep their missile defense systems in the country
Meanwhile, U.S. decides to establish a partnership with PKK’s Syrian armed wing YPG against ISIS. YPG seizes key cities along Turkish-Syrian border, creating concerns that YPG will be able to capture the whole border area. Turkey looks for an exit. Then comes Russia again
Turkey wants to clear ISIS from Jarablus, but it couldn’t do that. Because Russians were threatening Turkish jets. In fear of an emerging PKK state in its borders, Turkey decides to sign a reconciliation deal with Russia, in order to operate freely and seize the territory
As far as I gather from Turkish sources, Russians had a condition for the reconciliation deal: • Turkey would need to make a major Russian arms purchase as compensation for the downed jet and the killed pilot. Turkey takes the deal.
(The most important tweet which proves ruskies are nothing less in balls squeezing if you want help) 
For Turkish officials in Ankara, after the coup attempt in July 2016, S-400 seemed like a good solution for Russian demands. At the time; there was no CAATSA, US didn’t want to sell a defense system to Turkey, and no one brought up S-400s possible implications on F-35.
So until this point: • US/NATO didn’t ensure Turkey about threats posed by Russia in Syria • US partners with Ankara’s arch enemy. Turkey is fearful of a PKK state in itsborder • US doesn’t want to sell an air system to Turkey (They didn’t even want to sell smart munition)
So, July 2017. US learns that Turkey is going to purchase S-400s. What is the US talking point then? “How do you inter-operate in the NATO system?” asked then-US Defense Secretary Mattis. No mention of F-35
Btw, at this point Turkey had already signed the S-400 deal. • The deal was inked by April 2017 • Trump signs CAATSA in law, August 2017 US waits until November 2017 to issue a soft threat about F-35.
I was in Washington between 2014-2019. NOT A SINGLE US OFFICIAL I talked believed that Turkey would actually buy S-400: •“
It is a bluff” for concessions. • “I wouldn’t believe that for a second until I see with my own eyes that S-400s are actually in Turkey”
Turkish officials? They didn’t also believed that US was actually going to sanction Turkey over S-400z •
“It is a bluff” to make us buy Patriots • “They cannot force us out of F-35 program” So on and so forth. Both sides didn’t believe each other for a second.
I Believe this "it is a bluff" will surely repeat with USA and India too on S-400 
No genuine effort shown by both sides to resolve the S-400 crisis. Both thought the other was bluffing. • US was wrong to underestimate Turkey’s resolve. Failed to understand Turkey’s security concerns • Turkey needed to see that S-400 is a genuine threat to F-35
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Looks like we are dreaming same that US will sell us F-18 and possible F-35 after we get S-400. I see its dead end either we forget S-400 or We forget US tech . We are not going to get both . It didn't took much time for Turkey to go from trusted NATO ally to Pariah just because solely Turkey wanted to secure its interest .
What will happen to us if we want to secure our interest will US act the same way . Lets not forget Russia is not any loyal friends its basically a rouge Arms dealer who want to sell all its obsolete and advanced systems for price
"I am happy Ottaman is getting screwed as he always used to come to Aid of Paki pigs " But still the behaviour of USA is what we should be worried about
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Trump is giving Erdo a hard time since Erdo is being a lil bitch. They will loose F-35 contracts. However, this will give the US even a much closer look on the S-400.
US is looking at IN's SSN program, there have already been closed meetings.
