The revelation by Turkish President Recep Tayyip Erdogan that his government has paid Russia a deposit and signed an agreement for buying the S-400 anti-aircraft system is a setback for the US-led military bloc NATO. Despite internal disagreements and strains, no member state has until now broken the unwritten norm underpinning the alliance since 1949 of avoiding strategic defense cooperation with Russia. That Erdogan could contemplate such a bold démarche and undermine the existential anti-Russian logic of NATO says much about how dramatically the US is losing its grip over its allies.