The idea that one NATO ally might turn its military power against another was, until recently, almost unthinkable. The North Atlantic alliance was built on the assumption that threats would always come from outside its ranks. A hypothetical US move against Greenland — an autonomous territory within the Kingdom of Denmark — shatters that assumption and exposes a dangerous blind spot at the heart of NATO.
At the core of NATO lies Article 5, the mutual defence clause that treats an attack on one member as an attack on all. Its language is reassuringly simple when the threat is external, particularly from Russia. But the treaty never seriously contemplated what would happen if the aggressor were the alliance’s most powerful member. In such a scenario, the legal text offers little clarity and even less comfort.
That ambiguity was laid bare when Danish Prime Minister Mette Frederiksen warned that if the US were ever to attack another NATO country, the alliance’s machinery would effectively grind to a halt. NATO might continue to exist on paper, but its credibility — the very thing that deters adversaries — would be gravely damaged. The most obvious beneficiary of such paralysis would be Moscow, which has long sought to test and fracture Western unity.
The unease did not emerge in isolation. During the 2024 US election campaign, Donald Trump openly questioned America’s commitment to allies that failed to meet defence spending targets. Senior US officials later reinforced the message by signalling that Washington was no longer primarily focused on Europe’s defence. Although these remarks alarmed European capitals, the tensions were smoothed over ahead of the NATO summit through pledges to raise military spending and careful diplomatic choreography led by NATO secretary general Mark Rutte.
Yet the summit’s apparent success may have concealed, rather than resolved, deeper fractures. As analysts at institutions such as Chatham House have argued, flattery and tactical compromise are poor substitutes for a shared strategic vision. The uncertainty has already been visible in the alliance’s handling of Ukraine, where wavering US positions and pressure for territorial concessions have unsettled European partners.
Adding to the strain is a striking ideological shift in Washington’s outlook. Recent US strategy documents have framed Europe in stark, almost civilizational terms, questioning whether some NATO members will continue to see the alliance as their predecessors did in 1949. Against that backdrop, renewed talk of asserting control over Greenland has taken on a far more ominous tone, especially as it directly challenges the sovereignty of Denmark, a longstanding ally.
No one seriously expects NATO’s other members to come to Greenland’s military defence in such a confrontation. The imbalance of power is overwhelming, and US officials have been blunt in asserting that global politics is ultimately governed by force rather than treaties. NATO itself has no clear mechanism to expel a member, even one that violates the spirit of collective defence.
That is precisely why the damage would be so profound. An alliance built on trust cannot survive the spectacle of one member coercing another, even over a sparsely populated Arctic territory. Such an act would not merely weaken NATO; it would call into question the very idea of mutual security at a moment when external threats remain acute.
For European states still clinging to the belief that US security guarantees are immutable, the mere plausibility of a Greenland crisis should serve as a sobering wake-up call. NATO’s greatest strength has always been the assumption of solidarity. Once that assumption is broken from within, no amount of military spending or diplomatic reassurance can easily restore it.
