NATO, the world’s largest security alliance, typically concentrates on addressing threats from countries like Russia and dealing with international terrorism. The organization heavily relies on the leadership and military capabilities of the United States.

Established on a principle reminiscent of the “Three Musketeers,” NATO operates on a mutual defense commitment: an attack on one member is considered an attack on all. This promise, encapsulated in Article 5 of its founding treaty, has successfully deterred Russian aggression on allied soil for many years.

However, in an alliance that requires consensus, Article 5 becomes ineffective if one member nation acts against another.

Longtime NATO members and neighbors, Greece and Turkey, have a history of military confrontations and border disputes. Despite their tensions, these internal conflicts have never threatened the unity of NATO in the way that a hypothetical U.S. attempt to seize Greenland could.

On Wednesday, former President Donald Trump expressed his views on social media, stating, “RUSSIA AND CHINA HAVE ZERO FEAR OF NATO WITHOUT THE UNITED STATES.”

But he added: “We will always be there for NATO, even if they won’t be there for us.”

The White House took its threats toward Greenland to a new level on Tuesday, issuing an official statement that insisted Greenland is “a national security priority” and refusing to rule out the use of military force.

“The president and his team are discussing a range of options to pursue this important foreign policy goal, and of course, utilising the US military is always an option at the commander in chief’s disposal,” it said.

Ian Lesser, distinguished fellow at the German Marshall Fund of the United States think tank and an expert on NATO, described the White House statement as “very striking”.

“It’s a low-probability, high-consequence event if it were to happen. But the odds have changed, and so it becomes more difficult to simply dismiss this as bluster from the White House,” he said.

The statement came after the leaders of Britain, France, Germany, Italy, Poland and Spain defended the sovereignty of Greenland, along with Denmark, whose right to the island was recognised by the US government at the beginning of the 20th century.

“It is for Denmark and Greenland, and them only, to decide on matters concerning Denmark and Greenland,” the leaders said Tuesday in a joint statement. Canada, which sits off the western coast of an island that has been crucial to the defence of North America since World War II, expressed its support as well.

NATO itself remains reluctant to say anything that might annoy its leading member.

Danish Prime Minister Mette Frederiksen has warned that the US threat must be taken seriously, particularly after Trump ordered the capture of Venezuelan leader Nicolás Maduro in a night-time raid, and that any US attempt to take control of the island could mean the end of NATO.

Asked whether Frederiksen was right when she said that an American attack on another NATO country means that “everything stops,” an official at the alliance said: “NATO does not speculate on hypotheticals”.

The official, who spoke on the condition of anonymity because NATO protocol forbids the use of their name, preferred to note Greenland’s strategic significance.

“The Arctic is an important region for our collective security, and NATO has a clear interest in preserving security, stability and cooperation in the high north,” the official said.

“Together we make sure that the whole of the alliance is protected.”

Trump’s interest in Greenland also threatens to destabilise the alliance at the moment when the US-led efforts to end the war in Ukraine enter a pivotal stage, distracting its members from their efforts to support Kyiv and provide it with security guarantees.

Maria Martisiute, a defence analyst at the European Policy Centre think tank, warned that NATO’s credibility is on the line.

When a leading alliance member undermines another member, it hurts “NATO’s cohesion and credibility, and it serves only our adversaries such as Russia and China,” she said.

Rubio says he plans to meet Danish officials next week

US Secretary of State Marco Rubio said he plans to meet with Danish officials next week after the Trump administration doubled down on its intention to take over Greenland, the strategic Arctic island that is a self-governing territory of Denmark.

Rubio told a select group of lawmakers that it was the administration’s intention to eventually purchase Greenland, as opposed to using military force.

The remarks, first reported by the Wall Street Journal, were made in a classified briefing Monday evening on Capitol Hill, according to a person with knowledge of his comments who was granted anonymity because it was a private discussion.

On Wednesday, Rubio told reporters that Trump has been talking about acquiring Greenland since his first term.

“That’s always been the president’s intent from the very beginning,” Rubio said.

“He’s not the first US president that has examined or looked at how we could acquire Greenland.”

Rubio did not directly answer a question about whether the Trump administration is willing to risk the NATO alliance by potentially moving ahead with a military option regarding Greenland.

“I’m not here to talk about Denmark or military intervention, I’ll be meeting with them next week, we’ll have those conversations with them then, but I don’t have anything further to add to that,” Rubio said, telling reporters that every president retains the option to address national security threats to the United States through military means.

Tension comes after NATO leaders agreed to Trump’s demands

Last summer, NATO leaders rallied behind Trump’s demand that they increase defence spending. Apart from Spain, they agreed to invest as much per capita as the United States does, within a decade.

Just before Christmas, NATO Secretary-General Mark Rutte hailed Trump as a saviour.

“I believe fundamentally that thanks to Donald J. Trump, NATO is stronger than it ever was,” Rutte told BBC radio.

“NATO has never been as strong as this moment since the fall of the Berlin Wall.”

Yet in a year-end address in Germany meant to rally to European citizens behind defence spending, Rutte warned that Russia might attack elsewhere in Europe within a few years should it win in Ukraine.

“Conflict is at our door,” the former Dutch prime minister said. “Russia has brought war back to Europe, and we must be prepared for the scale of war our grandparents or great-grandparents endured.”

Lesser said it’s difficult to reconcile Trump’s defence spending victory with his designs on Greenland.

“What good is it to have revived NATO capability if it’s no longer a functional political alliance” afterwards? he asked. If that breakdown occurs, “it’s a gift to Moscow, and it’s a gift to Beijing.”