U.S. Trade Representative (USTR) Jamieson Greer has pressed the EU for more information regarding the $3.4 billion loan its lending arm, the European Investment Bank (EIB), made to Airbus in June, according to a statement Greer released Oct. 26. Greer said his office still has no details on what he called the bank’s “largest loan ever,” and warned it risks the “violation of the terms of our hard-won [Large Civil Aircraft] deal,” referring to the 2021 truce that paused tariffs on roughly $11.5 billion of goods. That truce was extended in July after the European Parliament approved the broader 15% tariff deal, so a fight over one Airbus loan could put both deals in play.

“There’s no way Europe backs away from this one. Airbus is real. It’s not a little sideshow,” Marc Busch, a professor of international business diplomacy at Georgetown University, told Fortune.

U.S. aircraft manufacturer Boeing requested the government press Airbus for transparency on the loan back in July, Reuters first reported. There was speculation at the time that Airbus was using the influx of cash to develop a new jet model.

“I had sincerely hoped that trade-distorting European subsidies to Airbus were a thing of the past,” Greer said in the statement. “For decades, France, Germany and Spain have aggressively supported Airbus, their national champion aircraft manufacturer, to gain an unfair advantage over U.S. aerospace workers and business.”

EIB did not respond to Fortune’s request for comment.

The U.S. brokered a trade deal with the EU last July, a pillar of President Donald Trump’s trade policy that set tariff rates at 15% and committed Europe to investing hundreds of billions of dollars in U.S. projects like energy and defense. That deal is now under threat because of an alleged violation of a truce between the U.S. and EU over aircraft subsidies violations at the World Trade Organization. 

The unwinding of the trade agreement could mean a reversion to a higher 30% tariff that has already weighed on American businesses and consumers, and the possible triggering of the EU’s massive retaliation package, which would impact more than $100 billion of U.S. goods.

Busch warns the reignition of an aircraft manufacturing subsidies dispute that was waged for years could indeed threaten transatlantic relations. Airbus provides 135,000 European jobs and about one-third of its revenue comes directly from Europe, so the EU will likely be quick to defend the government assistance it receives, Busch argued.

Airbus did not respond to Fortune’s request for comment.

“This is a big deal, and the next step will be tremendously important to get right,” Busch said.

How did the U.S. and EU’s aviation dispute begin?

The origins of this dispute over aviation subsidies go back to 1992, when the U.S. and EU minted an agreement allowing for government subsidies for aircraft production. By the next decade, however, Airbus deliveries had eclipsed those of its American rivals, and Boeing and the U.S. argued the European countries supporting Airbus were in breach of the WTO’s subsidies rules. That included a claim filed by the U.S. alleging Airbus saw a $200 billion economic benefit as a result of $22 billion in illegal subsidies. The EU launched a counterclaim that same day, alleging Boeing had received $23 billion in illegal support through tax breaks and defense contracts.

After years of court proceedings, WTO ultimately found in 2011 that both entities received subsidies that were either legally questionable, or violative by giving advantage to a manufacturer. The U.S. and EU both levied tariffs against each other, on $7.5 billion and $4 billion of goods, respectively, with the WTO’s permission.

In 2021, the U.S. and EU made a truce, that effectively ended the 17-year-long trade dispute—the longest dispute the WTO has ever heard. That truce was extended in July, continuing to suspect retaliatory tariffs.

What is the future of U.S. and EU trade relations?

The best-case scenario for the resolution of the alleged fuzziness around the $3.4 billion loan would be for EIB to open its books and say the loan resembles previous loans, and USTR puts the issue away, Busch said. But at its worst, the situation could escalate U.S.-EU tensions.

It’s not unlikely that should the Trump administration take issue with the nature of the loan, it could impose Section 301 tariffs on the EU in retaliation, and those tariffs could be under authorization of the WTO if the EU does indeed violate rules around aircraft subsidies. 

The situation alone would “amount to the ultimate irony of ultimate ironies,” Busch said. In 2020, the WTO ruled that Trump’s tariffs against China were illegal, and the president has repeatedly rebuked the organization, claiming it treats the U.S. unfairly.

“It would be quite something,” Busch said, “to imagine the ceasefire ends, we reimpose the Airbus tariffs, and Trump can—for the first time—get up in front of the camera and say these tariffs are WTO-authorized.”