Turkey’s purchase of a Russian air defense system looks set to frustrate the relationship between Ankara and its NATO partners. While other NATO countries are investing to free themselves of dependence on Russian defense equipment, it is unclear whether the purchase of the much-feared S-400 Triumf (SA-21 Growler) will simply be a one-off buy or the beginning of a new defense relationship between Ankara and Moscow. Under the $2.5 billion deal, signed on Dec. 29, Turkey will purchase ...