Being cynical is easy, but there is plenty of fairly high quality reporting on this issue in the mainstream media. https://www.reuters.com/article/us-usa-turke

Being cynical is easy, but there is plenty of fairly high quality reporting on this issue in the mainstream media. https://www.reuters.com/article/us-usa-turkey-trump/trump-says-not-fair-that-us-cant-sell-f-35s-to-turkey-idUSKCN1UB27J https://www.reuters.com/article/us-usa-turkey-trump/trump-sa... - “Turkish operation of the two systems together — and more specifically, when it is in stealth mode — would allow the S-400 to acquire intimate knowledge of the F-35’s radar signature,” Karako wrote. “Such insights would almost immediately find (their) way back to Russia, and the capability of F-35s around the world could thereby be degraded.” https://www.nytimes.com/2019/07/17/world/europe/us-turkey-russia-missiles.html https://www.nytimes.com/2019/07/17/world/europe/us-turkey-ru... - Ellen M. Lord, the Defense Department’s under secretary for acquisition and sustainment, said the S-400 and its radar systems could compromise the F-35’s stealth capabilities and jeopardize the fighter jet’s long-term security. https://edition.cnn.com/2019/07/16/politics/trump-turkey-s400-sales/index.html https://edition.cnn.com/2019/07/16/politics/trump-turkey-s40... - 'The S-400 poses a real threat to US national security, analysts said. The Russian system is built to defeat US stealth technology, leading officials in Washington to worry that if Turkey took delivery of F35s, the Russian system could be used to collect valuable information about the fifth-generation fighter jet.'