As US-Turkey ties continue their downward spiral, with both sides engaging in a tit-for-tat of sanctions and tariffs after Ankara refused to release an American pastor detained on terrorism charges, Turkey’s future in the F-35 program is being held hostage.

US President Donald Trump recently signed into law a defense spending bill that blocks the transfer of the fifth-generation fighter jet to Turkey. The legislation came just weeks after the first F-35 aircraft was delivered into Turkish possession, though the jet will remain on US soil for a training program.

While some lawmakers have brought up the case of the detained US citizen as one reason to hold up the F-35 transfer, the main concern cited by Congress was Ankara’s decision to purchase the advanced Russian S-400 missile defense system.

US Defense Secretary James Mattis has lobbied Congress to allow Turkey to remain in the program. An analysis published by the Australian Strategic Policy Institute on Tuesday explained why Mattis is concerned and why cutting Turkey out of the consortium will cause headaches for Australia and other participants.

“If Turkey is excluded from the JSF program, it would in turn cease providing components to the [F-35 Joint Strike Fighter] production line. That’s why Secretary of Defense James Mattis opposed the proposal, arguing in a letter to Congress that, ‘If the Turkish supply chain was disrupted today, it would result in an aircraft production break, delaying delivery of 50–75 F-35s, and would take approximately 18–24 months to re-source parts and recover,’” Marcus Hellyer wrote.

He added that Australia’s air force is on a tight timeline to get up and running with 33 F-35s within the next three years, and a two-year production delay would derail those plans.

Turkey was also assigned responsibility for engine deep maintenance in Europe, according to Hellyer, highlighting the degree of interdependence among the consortium members.

At the same time, Australia shares the US concerns about Turkey’s purchase of the Russian missile defense system.

“The JSF was designed to defeat high-end Russian air-defense systems such as the S-400. Russia will be very anxious to understand the JSF’s electromagnetic signatures, in particular, its radar profile. […]The Russian technicians who will be assisting the Turks to introduce and operate the S-400 will no doubt be collecting JSF signature data that can be used to develop tactics to defeat it,” Hellyer speculated in the article.

“In a ‘normal’ world one would expect the flow of information to go in the other direction, with Turkey sharing key signature and performance data on the S-400 with other NATO members. But when a NATO member is planning to acquire Russian air-defense systems, we’re not in a normal world anymore—not to mention Erdoğan’s threats to find new and different allies.”