In a letter sent to Turkish Defense Minister Hulusi Akar, Acting Defense Secretary Patrick M. Shanahan explained that the S-400 program is incompatible with the F-35 program, and that the two systems could not exist side by side.
"While we seek to maintain our valued relationship, Turkey will not receive the F-35 if Turkey takes delivery of the S-400," Shanahan wrote.
Still, Turkey may continue in participation in the F-35 program by shutting down procurement of the S-400 system, said Ellen M. Lord, the undersecretary of defense for acquisition and sustainment, during a June 7 news conference at the Pentagon.
"None of the steps we are taking are irreversible," Lord said. "If Turkey wants to stop procurement of the S-400, we would very much like them to continue in the F-35 program."
Shanahan's letter said U.S.-based training of Turkish personnel on the F-35 system would stop by July 31 if Turkey does not discontinue its S-400 purchase.
Additionally, Shanahan wrote that the U.S. will not plan for Turkish participation in the June 12 F-35 Chief Executive Officer Roundtable, and that planned updates to the F-35 program's governing documents will continue without Turkish input.
Lord explained in more detail how the U.S. would "begin unwinding Turkey's participation in the F-35 program" if plans for the S-400 purchase continue.
"If the United States and Turkey cannot reach a mutually agreeable resolution to this issue by July 31, all Turkish F-35 students and instructor pilots currently in the United States will be required to depart the country," Lord said.
Already, she said, a group of new students who were to arrive in June for training in the U.S. did not come for that training. Turkish students currently in training have not yet been asked to leave, however. "What we are trying to do is be respectful with the Turks as we move along," Lord said. "And we are hopeful that they will stop the acquisition of the S-400."
It's not just training on the F-35 that will cease if the Turkish don't discontinue their purchase of the S-400. The F-35 is an international program, and the aircraft itself is produced by multiple countries, including Turkey. Were the Turkish to proceed with the S-400 purchase, their involvement in F-35 production would also need to end.
"Turkey will receive no new work share in the F-35 program," Lord said. "Its current work share will be transitioned to alternate sources as they are qualified and come to rate production."
Lord said Turkish manufacturers are involved in building some 937 parts for the F-35, including many that make up the landing systems and the center fuselage. She said about 400 of those parts are sole sourced in Turkey — meaning there's no other partner manufacturing those parts.
For those parts of the F-35 being manufactured solely in Turkey, other manufacturers will need to be found. Lord said for now, that's going to be mostly U.S. manufacturers, and that the Defense Department is working with both Lockheed Martin and Pratt & Whitney to accomplish this.
The United States and other F-35 partners "have been working in earnest for the last six months to develop and implement changes to our supply base to accommodate the potential for Turkish suspension from the program," Lord said. "We are well underway" on that effort.
If S-400 procurement proceeds, Lord said, Turkish manufacturers will only be able to continue to make parts for the F-35 until sometime next year. Lord described it as "a wind-down in early 2020," a process not disruptive to the F-35 program, and one that "allows the Turks to wind down their activities as well."
The F-35 aircraft is designed to destroy weapons like the S-400, while the S-400 is designed to destroy weapons like the F-35. Because the two systems are meant to combat each other, Lord said, "we do not want to have the F-35 in close proximity to the S-400 over a period of time, because of the ability to understand the profile of the F-35 on that particular piece of equipment."
Lord said that the actions that will be taken to move Turkey out of the F-35 program are not a done deal, and that Turkey does have other options for air defense beyond the Russian-made S-400.
"The U.S. has been in active negotiations with Turkey over the sale of the Patriot air and missile defense systems since 2009, to satisfy its legitimate air defense needs," Lord said. "Should Turkey agree to suspend its S-400 acquisition, the U.S. is willing to partner with Turkey immediately to study ways to enhance Turkish security and address allied concerns with Turkey's S-400 purchase."
The U.S. goal, Lord said, is to protect the long-term security of the F-35 program as well as the capabilities of the NATO alliance.
"Turkey still has the option to change course," she said. "If Turkey does not accept delivery of the S-400, we will enable Turkey to return to normal F-35 program activities. Turkey is a close NATO ally, and our military-to-military relationship is strong."
Andrew Winternitz, deputy assistant secretary of defense for Europe and NATO, pointed out how Turkey might go about indicating their interest in continuing in the F-35 program.
"I think it would be great if they started winding down their acquisition of the S-400," he said. "I think, obviously, a good signal would be if they were to stop the training in Russia. That would be a great signal to us."