Donald Trump has presented NATO with an existential dilemma. The U.S. president has long been attacking the military organization. He has even gone so far as to question its very core, its fundamental principle: the mutual defense clause. Now, the White House’s threats against Greenland, an autonomous territory that is part of Denmark — a NATO member like the United States — without ruling out military action, could shatter the transatlantic alliance on which Europe has relied for its security since the end of World War II.

Danish Prime Minister Mette Frederiksen issued an unusually stark warning: “If the U.S. chooses to attack another NATO country militarily, then everything stops, including NATO and thus the security that has been established since the end of the Second World War.” But the wound has already been laid bare, and the rest of Europe is being forced to accept that the new order is here to stay.

The situation is so unusual that the treaties of the organization — which stipulate that its members must resolve their disputes peacefully — do not address what to do if one ally directly attacks another. Even less so if the aggressor is the most powerful member against one of the smaller ones — one that is strong and committed in terms of investment and participation in missions. Furthermore, the threat comes at a very sensitive time for Europe, with Russia’s war against Ukraine approaching its fourth anniversary and negotiations underway to end the conflict on European soil.

NATO continues to consider Russia its primary threat. However, the greatest danger now appears to lie within its own ranks. Would a U.S. incursion into Greenland truly spell the end of the Alliance? Jamie Shea, a high-ranking NATO official until 2018, is in no doubt. It would constitute a complete repudiation of the principles of the UN Charter, upon which the 1949 NATO Treaty is based. NATO (including the United States and Denmark) is currently responsible for Greenland’s defense and has incorporated this scenario into its regional defense plans. Therefore, the expert points out, a U.S. takeover of the island under the pretext of strengthening common defense against Russia and China in the North Atlantic region, as Trump has stated, would be a way of saying that NATO cannot do the job and that only the United States can guarantee the Alliance’s security. “That was the end of collective defense and respect for the sovereignty of NATO allies,” Shea, currently a researcher at the think tank Friends of Europe, points out.

Manuel Muñiz, rector of IE University in Spain and professor of international relations, echoes these sentiments: “Any military action against one NATO ally by another would irreversibly damage the Alliance; it would mark a turning point for the Alliance. I have no doubt about it,” he says.

Trump has already demonstrated in Venezuela that he is willing to resort to the use of force. After sending a Delta Force team to kidnap Nicolás Maduro, he stated: “We need Greenland from the standpoint of national security.” The White House has gone even further, making it very clear that all options are “on the table.”

In Europe, people are increasingly nervous. And that nervousness has also reached the heart of NATO. The issue has been raised in recent meetings, but for the moment, the decision has been made not to address it as a multilateral matter. The organization has long feared that the U.S. will eventually disengage from the Alliance and accelerate its withdrawal from Europe, where it is its main security umbrella. For this reason, sensitive topics are avoided. But this practice may now change. “The Greenland issue could become unavoidable,” says a NATO source, speaking on condition of anonymity. “The real question will be the reaction of the allies if Washington intervenes on the island, something that will also depend on the type of intervention.”

The White House believes Europe is not considering a scenario of defensive confrontation. “Nobody is going to fight the United States over the future of Greenland,” White House Deputy Chief of Staff Stephen Miller confidently told CNN in an interview last Monday. Nor would it be feasible. The United States has 1.3 million active military personnel across all branches; Denmark has 13,100. If the comparison is made in terms of defense spending, the disparity is also staggering, even when considering all allies: $845 billion in 2025 for the U.S., compared to a combined $559 billion for the other 31 NATO members.

“It’s difficult to imagine how the alliance would recover from such a shocking treaty violation as one ally attacking another to seize territory,” says Marion Messmer, director of the International Security program at Chatham House. “At the same time, European countries must seriously consider what NATO would look like without the U.S. and accelerate investments in those capabilities where the U.S. remains strongest, such as command and control networks, suppression of enemy air defenses, and similar enabling capabilities. They will also need to seriously consider what kind of adversary the United States might pose,” she says in an analysis. “Much of this should be done, and will be done, discreetly or privately. But states can no longer afford to ignore this possibility.”

It remains shocking that the question is being raised of how European allies could defend Denmark against another ally within the framework of the organization. And whether Article 5 of the collective defense treaty could be invoked (an attack on one ally is considered an attack on all, thus allowing them to respond). Experts believe it would not be applicable. Indeed, there is the precedent of the conflict between Turkey and Greece over Cyprus. When Ankara invaded the small island, the Alliance did not intervene, but the United States acted as a mediator.

To add fuel to the fire, the U.S. president has once again attacked NATO. On Wednesday, he escalated his rhetoric and sowed doubts about the common defense clause: “Everyone is lucky that I rebuilt our military in my first term and continue to do so. We will always be there for NATO, even if they won’t be there for us.” This only added to the unrest and discontent in Europe, which has done everything possible to appease him and lets his bluster pass without a response for that very reason.

If it comes to it, several European sources believe Trump will try to justify an intervention by arguing that it is essential for the security of the Arctic and the North Atlantic. The truth, however, is that bilateral agreements between Denmark and the U.S. allow Washington to significantly increase its military presence on the island and give it considerable leeway: “The United States already has sufficient guarantees and agreements with Denmark to increase its presence in Greenland as much as it wants and to reach economic and strategic agreements of all kinds. This [referring to a military intervention] appears to be a mere territorial claim. It would be an unmitigated act of aggression,” explains Muñiz of IE University.

“The message to Europeans will be that U.S. security can only be achieved by accepting total submission to Washington and turning a blind eye to U.S. violations of international law and exercises in realpolitik,” says Shea of Friends for Europe. “Few will believe that NATO could survive for long with that kind of transatlantic agreement. The headquarters and infrastructure in Brussels might remain for a while, but the alliance would have lost its meaning and its foundation.”

Messmer points out that NATO’s European allies would have several levers at their disposal if the U.S. continues its threats: they could refuse to resupply American ships in European ports; refuse to accept wounded military personnel for treatment in European military hospitals; and demand hefty payments for the continued stationing of U.S. troops. They could also propose closing certain military installations. “These measures were previously unthinkable. But they could reassure the United States that, while complaining about European ‘freeloaders’ on security has become popular, this has long been a mutually beneficial arrangement,” she concludes.

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