The chapter examines Turkey’s trajectory within the JF-35 program from its initial role as a key industrial and strategic partner to its eventual removal in 2019. Originally integrated into the F-35’s global supply chain, Turkey’s involvement was expected to modernize its air force while deepening transatlantic defense cooperation. Turkish firms produced critical components, and the country planned to purchase more than 100 F-35 jets. However, this partnership unraveled after the US deemed Ankara’s decision to acquire the Russian-made S-400 missile defense system incompatible with NATO systems. The chapter unpacks how the S-400 deal triggered the US to impose sanctions under CAATSA and ultimately expel Turkey from the program. It explores the political, economic, and strategic consequences of this decision both for Turkey and for the F-35 program. The resulting fallout revealed deep fractures in US-Turkey relations, with broader implications for NATO interoperability, defense industry cooperation, and strategic trust. The chapter also addresses Turkey’s subsequent turn toward alternative procurement strategies, including efforts to modernize its F-16 fleet, pursue the indigenous KAAN fighter, and diversify its partnerships. The case illustrates the limits of multilateral defense programs when national security priorities diverge.