NATO countries could attack another, but it is so far outside the alliance’s postwar imagination that its most famous clause does not clearly spell out what would happen if two members were to come to blows. Article 5 of the North Atlantic Treaty, the bedrock of collective defence, states that an armed attack against one or more allies in Europe or North America shall be considered an attack against them all. It is a simple formula when the threat comes from outside the alliance, particularly from Russia. It becomes dangerously ambiguous when the potential aggressor is NATO’s most powerful member.
That ambiguity has been thrust into the open by renewed talk in Washington of asserting control over Greenland, an autonomous Danish territory that hosts a key US military presence but remains under the sovereignty of Denmark, a Nato ally. While the prospect of a US invasion of Greenland still appears remote, the mere suggestion has shaken European capitals and raised existential questions about Nato’s credibility, cohesion and future purpose.
“If the US chooses to attack another Nato country, everything will stop,” Denmark’s prime minister, Mette Frederiksen, warned this week. Her remark captured the depth of anxiety in Europe: Nato might continue to exist as a legal entity, but its effectiveness as a mutual defence pact would be fundamentally compromised. The most obvious beneficiary would be Russia, which has long sought to weaken transatlantic unity and exploit divisions within the alliance.
The concern is not emerging in a vacuum. During the 2024 US election campaign, Donald Trump repeatedly questioned the value of Nato and openly threatened not to defend “delinquent” allies that failed to meet defence spending targets. At the time, the benchmark was 2% of GDP, a goal many European states struggled to reach. After Trump’s return to office, the rhetoric hardened. In February, his defence secretary, Pete Hegseth, declared that the US was no longer “primarily focused” on defending Europe, a statement that sent shockwaves through Nato headquarters in Brussels.
Diplomatic efforts in the run-up to Nato’s June summit appeared, at least on the surface, to calm the crisis. Under heavy pressure from Washington, allies agreed—Spain excepted—to raise defence spending to 3.5% of GDP by 2035. Nato’s secretary general, Mark Rutte, adopted a deliberately conciliatory tone towards Trump, praising US leadership in terms so effusive that critics accused him of excessive deference. The summit concluded with a show of unity, but many analysts now argue that it merely papered over deepening cracks.
“Yes, the summit went well in that Rutte found formulations that flattered Trump,” said Marion Messmer, a director at the Chatham House thinktank. “But I’m not sure how far that is a sustainable strategy.” The underlying problem, she and others argue, is not defence spending but a widening divergence in strategic outlook between Washington and its European allies.
That divergence has been particularly evident over Ukraine. For months, European governments have watched with unease as the US attempted—twice unsuccessfully—to pressure Kyiv into territorial concessions as a precondition for Russia even considering a ceasefire. The first attempt followed a summit in Alaska; the second involved the US endorsing elements of a Russian 28-point plan that would have forced Ukraine to give up more land. Both efforts failed, but they reinforced fears in Europe that Washington is prepared to strike deals over the heads of its allies in pursuit of a narrower conception of US interests.
Those fears were compounded in December by the release of a new US national security strategy that used unusually stark language about Europe’s future. Warning that the continent faced “civilizational erasure,” the document argued that demographic change would mean that within decades “certain Nato members will become majority non-European.” On that basis, it questioned whether these unnamed countries would continue to view their alliance with the US in the same way as the 12 founding members of Nato did in 1949. The passage was widely criticised in Europe as inflammatory and ideologically driven, but it underscored the sense that Washington’s commitment to the alliance was becoming conditional and transactional.
Against this backdrop, the re-emergence of US interest in Greenland has taken on outsized significance. The island, with a population of fewer than 60,000, sits astride the Arctic routes that are becoming increasingly important as climate change opens new sea lanes and intensifies competition for resources. The US already operates the Pituffik Space Base there, a crucial node in missile warning and space surveillance. But recent statements from senior figures in Washington have gone further, openly challenging Denmark’s historical sovereignty over the territory.
Few experts believe that Nato’s other 31 members would come to Denmark’s military aid if the US sought to seize Greenland by force. Trump adviser Stephen Miller made that point bluntly, arguing that the real world is “governed by strength, that is governed by force, that is governed by power”—not by treaties or notions of mutual obligation. Even if allies wanted to intervene, the imbalance of power would make it practically impossible.
The numbers are stark. The US maintains around 1.3 million active-duty military personnel across its services. Denmark has just over 13,000. Nato estimates show that Washington was expected to spend about $845bn on defence in 2025, compared with a combined $559bn for the other 31 allies. Recent US operations, including the rapid capture of Venezuela’s Nicolás Maduro and his wife, Cilia Flores, have been cited by American officials as demonstrations of the sheer scale and reach of US military power.
Legally, a US move against Greenland would plunge Nato into uncharted territory. The treaty contains no clear mechanism for expelling a member state, regardless of its behaviour. The preamble commits allies to “live in peace with all peoples and all governments” and to “safeguard the freedom, common heritage and civilisation of their peoples”—language originally intended as a safeguard against a member drifting into communism during the Cold War. Whether that wording could be invoked against a founding member turning on an ally is highly uncertain.
What is not in doubt is the damage such an act would inflict on Nato’s credibility. For 76 years, the alliance has been built on the assumption that its members would never use force against one another, and that the US would act as the ultimate guarantor of European security. A confrontation over Greenland, even without shots fired, would shatter that assumption and force allies to reconsider the value of collective defence guarantees.
Some argue that the damage is already being done. The combination of rhetorical threats, policy shifts and strategic ambiguity has, in their view, delivered a wake-up call to Europe at a time when the Russian threat feels more real than at any point since the Cold War. Despite being heavily embroiled in Ukraine, Moscow continues to probe Nato’s defences through cyber operations, disinformation campaigns and military pressure on its eastern flank.
“If any European states harbour any illusions they can rely on US security guarantees, then this is the wake-up call,” Messmer said. “We are not returning to that world.” Her assessment reflects a growing consensus in European capitals that, regardless of what happens over Greenland, Nato is entering a more uncertain and transactional era—one in which old assumptions about solidarity can no longer be taken for granted.
Whether the alliance can adapt to that reality without losing its core purpose remains an open question. What is clear is that the unthinkable—a Nato member threatening another—has already begun to reshape the strategic landscape of the North Atlantic.
