The Trump administration’s decision not to send cutting-edge fighter jets to Turkey could foreshadow a breakdown in military cooperation and even a U.S. blacklist against the key NATO ally due to its turn toward Russia.

Pentagon officials notified Turkey yesterday that they have stopped preparing to transfer the two F-35s Turkey has purchased, just days before this week’s summit in Washington commemorating the 70th anniversary of the transatlantic alliance. Turkish President Recep Tayyip Erdoğan has refused to cancel a deal to import Russian S-400 anti-aircraft missile defense systems, despite warnings from the United States that it would withhold the F-35s to prevent Russian specialists from learning how to track the stealthy warplanes.