The US Senate passed a defense spending bill recently that sought to prevent Turkey from getting 100 F-35 stealth jets, and now it could turn to Russia for the Su-57 if the bill becomes law.

Turkey has already agreed to buy one Russian weapons system, the S-400 missile defenses, which could give Moscow a window into NATO's defenses, which would be a nightmare for the alliance.

The Su-57 and Turkey's embrace of Russian military equipment both pose serious threats to NATO and its aircraft.

The US Senate passed a defense spending bill recently that sought to prevent Turkey from getting the 100 F-35 stealth jets it ordered — and now Turkey could become the first buyer of Russia's Su-57 "stealth" jet killer if the bill becomes law.

The US and Turkey have a number of ongoing diplomatic beefs, including Turkey claiming that the US is harboring clerics that urged a 2016 coup and detaining US citizens, and the US is claiming Turkey is a hub of illicit financing.

But on the military side, Turkey, a NATO ally, poses another threat by buying Russia's S-400 missile defense system.

The S-400, one of the most advanced missile defense platforms around, is meant to engage and shoot down US jets, like the F-35, that rely on stealth.

Retired US Air Force Lt. Gen. David Deptula told Business Insider that NATO countries "don't want to be networking in Russian systems into your air defenses" as it could lead to "technology transfer and possible compromises of F-35 advantages to the S-400."

If Turkey owned the F-35 and the S-400, it would give Russia a window into NATO's missile defense network and the F-35's next-generation capabilities. Basically, as NATO is an alliance formed to counter Russia, letting Russia patch in would defeat the purpose and possibly blunt the military edge of the most expensive weapons system ever built.

So for now, it seems selling the F-35 to Turkey is out of the question, leaving another option for Turkey — Russia's Su-57 "stealth" fighter jet.

"I don't want to be too cavalier here," said Deptula of the prospect of Turkey buying Russia's jets, "but what a joke. You got to be kidding me."

One or the other

Deptula cited the poor reputation of Russian aircraft, its maintenance records, reliability, and their ability to network with NATO systems as reasons why buying Su-57s wouldn't make sense. Deptula is not alone in disapproving of the Su-57, as India backed out of its cooperation in the project.

"If they bought the Su-57s and the S-400, those steps are incompatible with them buying the F-35. It's going to be either or," said Deptula. "And given the track record of Russian advanced aircraft, not a good move."

But despite the unproven nature of Russia's new fighter jet, a close review of recent photos of the plane by Business Insider revealed it's a direct competitor for US stealth jets like the F-22 and F-35.

So Turkey getting Su-57s would, for military and diplomatic reasons, present a nightmare scenario for NATO's security.

Correction: This article was updated to make it clear that the US has not yet passed a bill denying F-35 sales to Turkey.