The unresolved status of Turkey’s Russian S-400 air defense systems continues to block reentry into the F-35 program under U.S. law, specifically provisions in the National Defense Authorization Act and CAATSA that require certification Turkey no longer possesses the systems and will not acquire similar Russian equipment. President Trump signaled openness to deliveries during July 2026 talks in Ankara, yet Secretary of State Rubio stated in early October that statutory conditions must still be satisfied. Recent diplomatic contacts between Ankara and Moscow explore relocating the S-400s to a third country, while Turkey advances parallel Eurofighter procurement and receives U.S. approval for F110 engines supporting its domestic KAAN fighter. Bipartisan congressional opposition and the need for executive certification create ongoing procedural hurdles, with negotiations described by Turkish officials as progressing more slowly than anticipated.