NATO operates by consensus, so Turkish objections can stall nearly any policy, and its diplomats are both diligent and knowledgeable, on top of every ball, as one NATO official said. France has also used its effective veto to pursue national interests, but never to undermine collective defense, NATO ambassadors say. But Turkey has blocked NATO partnerships for countries it dislikes, like Israel, Armenia, Egypt and the United Arab Emirates.

More seriously, for many months Turkey blocked a NATO plan for the defense of Poland and the Baltic nations, which all border Russia. And Turkey wanted NATO to list various armed Kurdish groups, which have fought for their independence, as terrorist groups something that NATO does not do.

Some of these same Kurdish groups are also Washingtons best allies in its fight against Islamic State and Al Qaeda in Syria and Iraq.

A deal was supposedly worked out at the last NATO summit meeting in December in London, but Turkey created bureaucratic complications, and it was only in late June that Turkey relented after considerable pressure from official Washington, which has lost patience with Mr. Erdogan and is infuriated by his insistence on buying the S-400.

If deployed, the S-400 would put Russian engineers inside a NATO air defense system, giving them valuable insights into the alliances strengths while threatening to diminish the capability of the expensive fifth-generation fighter, the F-35.

The assumption is that Mr. Erdogan, who has grown significantly more suspicious since a failed 2016 coup against him, wants to be able to shoot down American and Israeli planes like the ones his own air force used in the coup attempt.

Every time we discuss Russia in NATO, everyone thinks of the S-400 and no one says anything, said one European diplomat, who spoke on the condition of anonymity to discuss a sensitive matter. Its a major breach in NATO air defense, and its not even discussed.

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Turkish Aggression Is NATOs Elephant in the Room - The New York Times