If Turkey buys the Russian S-400 air defense system, the Trump administration should restrict Turkey's purchase of the F-35 fighter jet.

Fortunately, as Valerie Insinna reports at Defense News, the U.S. is actively considering that potential response.

Still, the concern here is urgent in that Turkey's purchase of the S-400 is proceeding apace. The Turkish defense secretary claimed last weekend that the S-400 contract "is finished, the S-400 missiles have been bought. The rest is just details now."

"If so," the U.S. should respond, "forget buying the F-35."

This isn't that complicated.

First off, the fact that President Recep Tayyip Erdogan is even contemplating taking delivery of advanced Russian military equipment is outrageous. As a member of NATO, Turkey is expected to buy military capabilities that can work seamlessly alongside other NATO member militaries. The S-400 does not allow for that operational synergy. On the contrary, this purchase represents a betrayal of NATO's founding mission: to deter and defeat any Russian attack on Europe or the United States.

In turn, while the U.S. likely has electronic warfare capabilities to defeat the S-400, these obviously now cannot be shared with the Turkish military. The need is simple in that the F-35 is designed to penetrate modern air defense networks. The F-35s purchase by Turkey alongside the S-400 would obviously be a great pathway to its own redundancy.

Don't get me wrong, I recognize that cutting Turkey out of the F-35 program would be a major step. After all, Turkey has been a participant in the program since 2002 and intends to purchase more than 100 of the aircraft. Nevertheless, U.S. and allied national security interests require that no nation have the F-35 if they are also utilizing Russian military capabilities that would undercut the program. As Insinna notes in her Defense News piece, were Turkey to possess both the S-400 and the F-35, it could simply fly the F-35 around and figure out how best to target it with the S-400.

At that point, in light of Erdogan's increasing submissiveness to Russian President Vladimir Putin, the U.S. would face very real concerns of Turkey handing over its targeting data to Russia. That risk would pose an intolerable danger to our aircrews and to the NATO mission of effective warfighting.

Ultimately, Trump should tell Erdogan that he has two choices here. The Turkish leader can go ahead and take delivery of the S-400 or he can take delivery of the F-35. But he cannot have both.