This has always been a serious concern—that Turkey would use the S400 against US or NATO allies. The other concern being that if connected to other NATO systems, the S400 could pose a major interoperability (cyber) threat.
And I am hearing from Greek friends that the Turks may have done just that: connected S400 to NATO radar systems.

If all of this can be confirmed by a US official, a swift response will be necessary. Nothing less than Turkey’s NATO membership should be at stake.
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