Russia and Turkey brace to close a strategic deal that is controversial by NATO standards; Ankara is about to buy the latest S-400 air-defense system.

The technical side of the contract is nearing completion, according to a statement by the S400 system manufacturer, Rostec, on Saturday, June 3. Turkey will be the third buyer of the system after India and China.

Speaking on the sidelines of the of the St. Petersburg International Economic Forum (SPIEF), Rosetec’s CEO, Mr. Chemezov, confirme...