European Commission President Ursula von der Leyen delivered a blunt warning Tuesday, calling President Trump’s threat to slap tariffs on eight NATO allies over Greenland a “mistake” that could send U.S.–European relations into a dangerous free fall.

Speaking at the opening of the World Economic Forum in Davos, Switzerland, von der Leyen said the proposed tariffs risk plunging relations between longtime allies into a “downward spiral” at a moment when unity over Arctic security is more critical than ever.

“When it comes to the security of the Arctic region, Europe is fully committed and we share the objectives of the United States,” von der Leyen said, pointing to Finland—NATO’s newest member—as proof. She highlighted Finland’s recent move to sell icebreakers to the U.S., saying it demonstrated that Europe’s northern allies already have Arctic-ready forces in place.

“That Arctic security can only be achieved together,” she said. “And this is why the proposed additional tariffs are a mistake, especially between long-standing allies.”

Von der Leyen reminded the audience that the European Union and the United States reached a trade deal just last July. “In politics, and in business, a deal is a deal,” she said. “And when friends shake hands, it must mean something.”

The dispute centers on Greenland, a semi-autonomous territory of Denmark for more than 70 years that has increasingly become a flashpoint in Arctic geopolitics. Denmark’s prime minister, Mette Frederiksen, warned lawmakers Tuesday that tensions with the Trump administration could still worsen.

“We are now being threatened by our closest ally,” Frederiksen told Denmark’s parliament, adding that “the worst may still be ahead of us,” according to the Associated Press.

Von der Leyen said the EU’s 27 member states view Americans not just as allies, but as friends—and that a trade confrontation would only serve shared adversaries. “Plunging us into a downward spiral would only aid the very adversaries we are both so committed to keeping out of the strategic landscape,” she said, promising that Europe’s response would be “unflinching, united and proportional.”

She announced that the EU is working on a new package of measures to bolster Arctic security, with “full solidarity with Greenland and the Kingdom of Denmark” as its first principle. “The sovereignty and integrity of their territory is non-negotiable,” von der Leyen said.

The plan, she added, would include support for infrastructure in Greenland and expanded cooperation with Denmark and other partners—while still seeking collaboration with the United States on broader Arctic security. “This is clearly in our shared interest, and we will step up investment,” she said.

Greenland’s prime minister, Jens-Frederik Nielsen, who did not attend Davos, echoed that message from afar. His government, he said, has been meeting with NATO members and allies and believes Western unity must be grounded in “respect for national, territorial integrity (and) respect for international law.”