Despite opposition from Congress, Turkey will receive its first F-35 Joint Strike Fighter jets from the Pentagon's top weapons supplier on Thursday.
Following a formal handoff ceremony at Lockheed Martin's F-35 facility in Fort Worth, Texas, the defense giant will ferry the aircraft to Luke Air Force Base in Arizona where Turkish pilots will begin training alongside U.S. airmen.
The two fifth-generation jets are the first of what the NATO member and F-35 program partner hopes will be the start of a 100-strong fleet.
However, both House and Senate versions of the 2019 National Defense Authorization Act contain restrictions on Turkey's participation in the F-35 program due to Ankara's plan to buy Russia's advanced S-400 air defense system.
On Tuesday, the Senate voted 85 to 10 to block the sale, citing the deal with the Russians as well as Turkey's continued detention of American pastor Andrew Brunson, who was accused of spying and attempting to overthrow the government in 2016. He has denied all charges.
Turkish Prime Minister Binali Yildirim called the Senate vote "lamentable," describing it as against the spirit of NATO. He added that his country was "not without alternatives."
The Russian-made missile system, which is equipped with eight launchers and 32 missiles, is capable of targeting stealth warplanes like the F-35 fighter.
"The Turkish government claims to have purchased a Russian air defense system designed to shoot these very planes down," Sen. Jeanne Shaheen, D-N.H., said last week on the Senate floor. "We would be handing this technology over to the Kremlin if we granted Turkey these planes, and Congress will not stand for it."
What's more, the air defense system's radar could reveal vulnerabilities in the jet that Turkey could then relay to Russia. The conflict of interests among NATO partners is proving a growing headache for the alliance.
"There is a general concern across the NATO alliance, and certainly within the Department of Defense and now in Congress, that the Turkish purchase of an S-400 would allow the Russians to have a backdoor into very hyperactive radar readings of the alliance's front-line jet for decades," Aaron Stein, a senior fellow at the Atlantic Council, told CNBC.
"If you kick Turkey out of the F-35 program, you're basically saying that they can't be trusted with this fighter jet and that calls into question the NATO alliance."
Todd Harrison, senior fellow and director of defense budget analysis at the Center for Strategic and International Studies, noted that it would likely take months before the Senate's measure became law.
"So in the meantime, the administration can move ahead with the sale because it is not yet prohibited in law," Harrison told CNBC.
'A really nasty fight'
But things could get dicey later down the road — in the next eight to 12 months, when the S-400 is set to be deployed in Turkey.
"There could be some efforts to block funding or the transfer of those jets out of the U.S. and into Turkey," Stein said.
"I would say we are looking at an eight-month timeline to get this resolved, otherwise we are going to come down to a really nasty fight over the transfer of the jets to Turkey."
In response to questions over the S-400 purchase, Turkish President Recep Erdogan has remained defiant. "When it becomes necessary, we will use the S-400," he told local media earlier this month. "What are we going to do if we don't use the defense system — are we going to rely on the United States?"
The Pentagon did not immediately respond to CNBC's request for comment.