The difficult relationship between NATO and Turkey is facing yet another challenge. The Turkish government now plans to welcome deliveries of Russian S-400 air defense systems in 2019, along with the arrival of Lockheed Martin’s F-35s. In 2002, Turkey signed on as one of the earliest partners on the F-35 program, committing to the purchase of 100 fighters. In the process, Ankara has earned F-35 maintenance work to the tune of $12 billion. But the diplomatic relationship between NATO ...