In March 2025, a scene unfolded outside the American consulate in Nuuk, Greenland, that would have been unthinkable just years before. Protesters gathered to demonstrate against threats from the United States, a fellow NATO ally, highlighting a profound crisis brewing at the heart of the transatlantic defence pact.
The Unthinkable Scenario: An Ally Turns Aggressor
The core principle of NATO, enshrined in its famous Article 5, states that an armed attack against one member in Europe or North America is an attack against all. This mutual defence clause was designed to deter external threats, primarily from the Soviet Union and now Russia. It contains no clear provision for what happens if the aggressor is another member state.
This theoretical weakness has been thrust into stark reality by the re-emergence of US territorial interest in Greenland, an autonomous territory of Denmark. The notion, famously floated by former President Donald Trump and revived by his administration, challenges the historical sovereignty of a fellow NATO ally. As Danish Prime Minister Mette Frederiksen stated, if the US chose to attack another NATO country, "everything will stop."
The alliance might continue to exist, but its foundational promise of collective security would be shattered. The obvious beneficiary of such a fracture, analysts warn, would be an already aggressive Moscow.
Papering Over the Cracks: NATO's Precarious Diplomacy
Diplomatic efforts, particularly ahead of the June 2025 NATO summit, appeared to smooth over tensions. Under Secretary General Mark Rutte, who was criticised for overly flattering remarks towards the US President, allies agreed to raise defence spending to 3.5% of GDP by 2035. However, experts believe this simply masked a deeper rift.
"Yes, the summit went well in that Rutte found formulations that flattered Trump. But I’m not sure how far that is a sustainable strategy," said Marion Messmer, a director at the Chatham House thinktank.
The underlying tensions have been evident for months, notably in transatlantic disputes over Ukraine and in the extraordinary language of the US national security strategy. That document warned Europe faced "civilizational erasure" and questioned whether future, more demographically diverse NATO members would view the alliance as its founders did in 1949.
A Stark Power Imbalance and the End of Illusions
The sheer disparity in power makes the Greenland scenario particularly chilling for the alliance. The US is expected to spend $845bn on defence in 2025, compared to a combined $559bn from the other 31 allies. With 1.3 million active personnel, the US military dwarfs Denmark's 13,100.
As Trump adviser Stephen Miller asserted, the real world is "governed by strength... governed by force, governed by power"—not treaties. No other NATO member would be expected, or able, to militarily defend Greenland against a US seizure.
While there is no clear mechanism to expel a member from NATO, an attack by one ally on another—even over an Arctic territory with under 60,000 people—would irrevocably undermine the 76-year-old alliance's credibility. "If any European states harbour any illusions they can rely on US security guarantees, then this is the wake-up call we are not returning to that world," Messmer concludes.
The protests in Nuuk are more than a local grievance; they are a symbol of a fundamental crisis of trust that strikes at the very heart of the West's primary defence alliance, leaving it vulnerable at a time when the Russian menace has never felt more acute.
