In a stark demonstration of rising transatlantic tensions, protesters gathered outside the American consulate in Nuuk, Greenland, in March 2025, rallying against perceived US threats to the vast Arctic territory. The scene underscores a previously unthinkable geopolitical scenario now being debated: a potential US challenge to Danish sovereignty over Greenland, an action that experts warn would catastrophically undermine the Nato alliance from within.
The Article 5 Conundrum: When the Attacker is an Ally
The cornerstone of the North Atlantic Treaty Organisation is its mutual defence clause, Article 5. It states clearly that an armed attack against one member in Europe or North America "shall be considered an attack against them all." This principle has been a deterrent against external aggression for 76 years. However, the alliance's founding treaty contains no clear provision for what happens if the aggressor is another member state—specifically, its most powerful one.
Danish Prime Minister Mette Frederiksen articulated the crisis bluntly on Monday, stating, "If the US chooses to attack another Nato country, everything will stop." While the institution might technically survive, its credibility and operational effectiveness would be shattered. The primary beneficiary of such a rupture, analysts concur, would be an already aggressive Moscow.
From Rhetoric to Reality: The Buildup to a Crisis
The current strain did not emerge overnight. During the 2024 US election campaign, Donald Trump reiterated his stance that America would not protect Nato members he deemed "delinquent" for failing to meet defence spending targets. This sentiment was echoed by his Defence Secretary, Pete Hegseth, in February, who emphasised the US was no longer "primarily focused" on defending Europe.
Although the subsequent Nato summit in June 2025 appeared to paper over the rift—with Secretary General Mark Rutte's diplomatic flattery and a new agreement for allies to raise defence spending to 3.5% of GDP by 2035—the underlying divisions remained profound. Marion Messmer, a director at the Chatham House thinktank, noted the summit's success was superficial, relying on formulations that "flattered Trump," and questioned the sustainability of such a strategy.
The re-emergence of US territorial interest in Greenland, particularly following the high-profile capture of Venezuela's Nicolás Maduro, has brought this theoretical vulnerability into sharp, alarming focus. The US explicitly challenging the sovereignty of Denmark, a fellow Nato ally, moves the crisis from diplomatic posturing to a direct confrontation of the alliance's principles.
An Unbalanced Fight and a Broken Trust
The military imbalance makes any collective defence against US action on Greenland a fantasy. The US is projected to spend $845 billion on defence in 2025, compared to a combined $559 billion from the other 31 Nato allies. With 1.3 million active personnel, the US military dwarfs Denmark's force of 13,100. The operation to capture Maduro served as a recent, potent demonstration of this sheer, unilateral power.
As Trump adviser Stephen Miller asserted, the real world is "governed by strength... force... power"—not treaties. No other ally would be expected, or able, to militarily defend Greenland. Furthermore, the Nato treaty lacks a clear mechanism for expelling a member, even one that attacks another. While the preamble commits members "to live in peace with all peoples," this wording is largely symbolic.
The damage, however, would be irreparable. An attack by one member on another, even over a remote Arctic territory of under 60,000 people, would destroy the foundational trust upon which the 76-year-old alliance is built. Messmer argues that the mere discussion of such threats serves as a definitive "wake-up call" for European states, proving they can no longer rely unquestioningly on US security guarantees, especially with the Russian menace feeling more real than ever.
The standoff over Greenland is more than a bilateral dispute; it is a stress test for the entire architecture of transatlantic security, revealing a critical flaw that could be exploited to collapse the alliance from within.
