In a visit to the Kremlin on Monday, Turkish President Tayyip Recep Erdogan reiterated his determination to press ahead with the purchase of a Russian missile defense system that has strained ties with the United States.

“We have determined our road map for the S-400,” Erdogan said. “No one can expect us to give up on this.”

Not to be outdone, the Republican and Democratic leaders of the Senate foreign affairs and armed services panels stated unequivocally in a New York Times op-ed today that US sanctions on Russian arms makers preclude the delivery of the US F-35 jet if Ankara moves ahead with the deal to buy the S-400, which the Pentagon fears Russia could use to spy on its aircraft. “By the end of the year,” the senators wrote, “Turkey will have either F-35 advanced fighter aircraft on its soil or a Russian S-400 surface-to-air missile defense system. It will not have both.”

Why it matters:  Turkey’s increasing closeness with Russia has been driving Washington and Ankara apart on a number of fronts, including military sales and negotiations over the future of Syria. Here’s a look back at how the S-400 deal and other irritants have impacted the relationship.