The U.S. State Department notified Congress of a proposal to sell the Patriot air and missile defense system to Turkey, a move that may signal a breakthrough with a NATO ally that had been moving closer to Russia and roiling administration plans for the F-35 fighter jet, Bloomberg reports.

The potential $3.5 billion deal for the Patriot system, which is manufactured by Raytheon Co. and Lockheed Martin Corp., marks an opening gambit by the Trump administration to get Turkey to halt its purchase of Russia’s S-400 missile system, which the administration and Congress had vehemently opposed, according to four people familiar with internal discussions before the announcement.

Tensions between the U.S. and Turkey have escalated in recent years as the government of President Recep Tayyip Erdogan turned more authoritarian and the two allies disagreed over U.S. support for Kurdish militias in Syria. In recent days, Erdogan threatened to send his forces in Syria toward Kurdish-held areas where U.S. troops are operating, raising the prospect of a battlefield confrontation between NATO’s two biggest armed forces.

“It’s a big deal, because only a few months ago, analysts were predicting all doom and the end of a nearly seven-decades-old U.S.-Turkey relationship,” said Soner Cagaptay, the director of the Turkish research program at the Washington Institute for Near East Policy. “This suggests a new phase in U.S.-Turkish ties, one driven by a new rapport between Presidents Erdogan and Trump, and aided by bureaucracies in both countries determined not to let the U.S.-Turkish relationship collapse.”

Turkey is a crucial partner in the U.S.-led program to build the F-35 stealth fighter jet, with 10 Turkish companies set to produce about $12 billion in parts, including key components such as the center fuselage and some landing gear.

The Russian S-400 was designed to shoot down U.S. and coalition aircraft at greater ranges and altitudes than older systems. U.S. officials are concerned that sensitive F-35 technology designed to evade such a system could be compromised and used to improve the Russian air defense system if Turkey takes possession of both systems.