The acting head of the Pentagon has sent a letter to the Turkish defense minister, detailing the US plan to exclude Ankara from the F-35 stealth fighter jet program over its acquisition of the Russian S-400 air defense systems.

Acting US Secretary of Defense Patrick Shanahan has informed his Turkish counterpart, Hulusi Akar, how the US will be extracting Turkey, a major partner and sole supplier of dozens of parts for F-35 jets, from the program.

The letter dated June 6 contains an ‘Unwinding Turkey’s Participation in the F-35’ chart, according to which Turkey must cease all operations related to the program by July 31. That includes training of its pilots, who will be allowed to remain in the US up to that date. Past the deadline, they will be barred from entering Luke Air Force Base in Arizona and Eglin Air Force Base in Florida, where the training is taking place, and their invitations rescinded. Currently, 42 Turkish trainees are stationed in the bases.

Confirming earlier media reports, Shanahan wrote that the US will not accept any new pilots, “because we are suspending Turkey from the F-35 program.”

Turkey will not be invited to the annual F-35 CEO roundtable on June 12 and will not get a scheduled production, sustainment, and follow-on development update on the program. The US will “suspend indefinitely” deliveries of material and equipment to Turkey within the scope of the program.

Turkey’s roadmap out of the F-35 project comes right after Russian state defense corporation Rostec announced on Friday that the first S-400 anti-air missile system batteries will be arriving in Turkey in two months, ahead of the initial schedule.

Rostec CEO Sergey Chemezov indicated that Russia had already completed training of the Turkish specialists to man the cutting-edge surface-to-air defense complexes.

The purchase of the Russian-made systems has derailed Turkey’s plans to buy 100 F-35 jets. Washington considers the presence of the S-400s in Turkey to be a security threat to the F-35s and NATO as a whole.

In his letter to the Turkish official, Shanahan reiterates the ultimatum, noting that Turkey “will not receive the F-35 if… it takes delivery of the S-400.” In addition to repeating claims that S-400s would compromise NATO interoperability and the security of the F-35 warplanes, Shanahan says the deal, struck between Ankara and Moscow in 2017, will “lead to Turkish strategic and economic over-dependence on Russia,” presumably forcing the US out of that niche.

Painting a grim picture of Turkey without F-35s, Shanahan predicts “a loss in jobs, gross domestic product, and international trade.” On top of that, the US may also hit Turkey with sanctions under the Countering America’s Adversaries Through Sanctions Act (CAATSA). All Ankara has to do to avoid trouble is ditch the deal with Russia, he writes – something Ankara has refused to do time and again, insisting the S-400s are a more efficient solution for its defense than the alternatives offered by the US.

Apart from planning to buy F-35s, Turkey is also a key link in its supply chain, producing a total of 937 parts, 400 of them exclusively, and investing over $1 billion in the project. Undersecretary of Defense for Acquisition and Sustainment Ellen Lord confirmed on Friday that the US aims to remove Turkish companies from the F-35 loop by “early 2020,” maintaining that there would be “no major disruptions and very few delays,” despite the loss of Ankara’s huge contribution to production.