The Turkish Defense Ministry says the first components of a Russian-made S-400 anti-aircraft missile system have arrived, something Washington had hoped to stave off by threatening to withhold delivery on the 100 F-35 advanced stealth fighter jets Ankara had ordered. The US is concerned that the Russian presence in Turkey – the air-defense system will be assembled and most probably operated, at least in the beginning, with the assistance of Russian technicians – could compromise some of the F-35’s stealth capabilities if the jets are used to fine-tune the S-400. (Complicating things for Lockheed Martin, the jet’s manufacturer, is the fact that Turkey currently produces hundreds of its parts, and canceling that sale could pose a supply-chain problem, slowing down production for other clients.) Turkey’s use of the S-400 also upends a rule in NATO that the military systems of member-states be compatible with one another. The S-400 cannot be used in conjunction with the systems of other treaty members. Ankara has belonged to NATO since 1952.