How a US takeover of Greenland would undermine Nato from within

The idea that one Nato country could attack another – a US invasion of Greenland – is so alien that the most famous article in Nato’s founding treaty does not distinguish clearly what would happen if two of its members were at war.

Article 5, the cornerstone of mutual protection, dictates that “an armed attack against one or more” in Europe or North America shall be considered “an attack against them all”. Simple enough if the military threat comes from Russia, but more complicated when it comes from easily the alliance’s most powerful member.

“If the US chooses to attack another Nato country, everything will stop,” Denmark’s prime minister, Mette Fredriksen, said on Monday. The military alliance may well continue to exist but its effectiveness will be called into fundamental question; the obvious beneficiary, an already aggressive Moscow.

During the 2024 election campaign, Donald Trump said he would not protect “delinquent” Nato members, countries that did not meet the then target of spending 2% of GDP on defence. The US was no longer “primarily focused” on defending Europe, his defence secretary, Pete Hegseth, emphasised in February.

It was enough to provoke alarm in Europe, but diplomacy in the run-up to June’s Nato summit appeared to have massaged away the problem. Leavened by the unctuous comments of the secretary general Mark Rutte – he called the US president “daddy” – Nato allies, bar Spain, agreed to lift defence spending to 3.5% of GDP by 2035.

Yet, rather than heal differences in opinion, it appears the Nato summit simply papered over a rift. Marion Messmer, a director at the Chatham House thinktank, says: “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.”

There have already been several months of transatlantic uncertainty about Ukraine caused by two failed US efforts to force Kyiv, after the Alaska summit and again with the adoption of the Russian 28-point plan, to give up more territory as a precursor to the Kremlin even considering a ceasefire.

December’s US national security strategy hectored Europe, with its extraordinary warning that the continent faced “civilizational erasure”, partly because, within a few decades, “certain Nato members will become majority non-European”. On that extreme basis, the strategy questioned if these unnamed countries would view their alliance with the US “in the same way” as did the 12 who founded Nato in 1949.

If the diplomatic dance and the noises were not clear enough, then the re-emergence of the territorial lust for Greenland in the aftermath of the capture of Venezuela’s Nicolás Maduro has finally brought Nato itself sharply into focus, with the US explicitly challenging the historical sovereignty of Denmark, a fellow ally.

Nobody would realistically expect any of Nato’s 31 other members to defend Greenland militarily if the US sought to seize it, a point emphasised by Trump’s adviser Stephen Miller overnight. The real world, he added, was “governed by strength, that is governed by force, that is governed by power” – not treaties or mutual support.

Nor would they have any hope of doing so. The US has 1.3 million active military personnel, across all its services; Denmark 13,100. Nato figures show the US was expected to spend $845bn on defence in 2025, the other 31 allies a combined $559bn. The ease with which the US was able to capture Maduro and his wife, Cilia Flores, is a demonstration of the scale of sheer American power.

The alliance’s membership may not even change even if the US did take Greenland. There is no clear provision in the Nato treaty for expelling a country, though its preamble does commit the US and other allies “to live in peace with all peoples and all governments” and “to safeguard the freedom, common heritage and civilisation of their peoples” – wording once intended to be used against a member that became communist during the cold war.

Nevertheless, one alliance member turning on another, even over an Arctic territory with a population of less than 60,000, would undermine the credibility of the 76-year-old military alliance, intended to ensure peace and mutual protection across Europe and the North Atlantic.

Even the latest round of threats, some argue, has caused enough damage at a time when the Russian menace has never felt more real, even if Moscow is currently heavily embroiled in Ukraine. “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 says.