(Not sure if this is the right place for the question, but the roots of it are political.)
From what I've seen, Turkey's decision to buy the Russian S-400 anti-air system has jeopardized its relationship with its NATO allies, and earlier today the US went further and decided to stop training Turkish pilots on how to fly the F-35. I don't understand why. I can see why Turkey choosing to buy a rival's weapons are an embarrassment to NATO, but I don't see why we have quotes like this (from the source):
Kathryn Wheelbarger, one of the Pentagon’s most senior policy officials, said last week that Turkey’s completion of the transaction with Russia would be "devastating," dealing heavy blows to the F-35 programme and to Turkish interoperability within the NATO alliance.
"The S-400 is a Russian system designed to shoot down an aircraft like the F-35," said Wheelbarger, an acting assistant secretary of defense. "And it is inconceivable to imagine Russia not taking advantage of that (intelligence) collection opportunity."
- Why would Turkey possessing the S-400 deal a heavy blow to the F-35 programme? If anything, it would be a security risk for Russia, since now a NATO country will have a detailed idea of how the S-400 works, how dangerous it is, what its limitations are, and so on.
- Even if we assume that Turkey starts testing how effective the S-400 is at shooting down the F-35, it still doesn't explain how Russia is going to get hold of that intelligence.
- Even if Russia can get hold of that intelligence, it doesn't explain why refusing to train Turkish pilots on how to fly the F-35 does anything (unless this is completely a political tool to pressure Turkey). In other words, does Turkey possessing the S-400 but not the pilots to fly the F-35 accomplish anything?
If we leave aside the political embarrassment, why would Turkey possessing the S-400 be a problem for NATO?