As the National Defense Authorization Act (NDAA) makes its way through the House and Senate Armed Services Committees, the amendments in response to Turkey’s pending purchase of the Russian S-400 air missile defense system are creating a flashpoint in an already tenuous relationship with the NATO ally.

In SASC’s version of the NDAA—which allocates all spending for the Department of Defense—the committee directed the Pentagon to create and present a plan to Congress to remove Turkey from the F-35 program. In the HASC version, an amendment was passed that would halt all weapons sales, including the F-35, to Turkey, until the DoD reports on the status of the US alliance with Turkey.

Turkey responded, vowing to retaliate if the final NDAA bill stops the sale of the Lockheed Martin-made F-35 jets. The two versions of the NDAA from HASC and SASC will be merged into one over the summer and then must make its way through both chambers again before going to President Trump’s desk.

The proposals come after increasing tensions with Turkey, who some US officials claim has been cozying up to Russia at a time where nearly all top brass and civilians military leaders are echoing the National Defense Strategy and its unambiguous point to great power conflict with Russia and China.  

In December, Turkey announced it had reached an agreement with Russia to purchase two S-400 long-range air missile defense systems by early 2020. The S-400 is not interoperable with NATO missile defense systems.

“No partner in NATO has ever purchased a Russian system that is not capable of being interoperable with our NATO systems,” U.S. Ambassador to NATO Kay Bailey Hutchison said last week ahead of the annual NATO summit to be held in mid-July.

Senator Jeanne Shaheen (D-NH), who proposed SASC’s amendment halt of F-35 sales to Turkey, said “There is tremendous hesitancy [about] transferring sensitive F-35 planes and technology to a nation who has purchased a Russian air defense system designed to shoot these very planes down.”

In 2014, Turkey placed an order for the first two F-35 jets of a projected fleet of 100 F-35A aircraft with plans to deploy the aircraft by 2019. The US was expected to begin delivery of the F-35 planes later this year, with the first shipment scheduled for June 21.

“We would not want to have [the F-35] close to the S-400,” Secretary of the Air Force Heather Wilson said at a talk on strengthening alliances at the Atlantic Council. “We always want our allies to have equipment that is interoperable … but sometimes it’s the United States that’s part of the problem.”

Wilson pointed to US export controls, which she claims can force our allies into a situation where they buy defense systems from other great powers, like China or Russia.

Despite this, Wilson said she’s “hopeful” the situation will be resolved.

“We can compete and deter and win most effectively when we operate together with our allies,” she said. “In general [that] means building a foundation of mutual respect of shared responsibilities and accountability to each other.”