The idea of a NATO member attacking another NATO member is supposed to be unthinkable. For more than seven decades, the alliance has survived Cold War tensions, internal disputes, and political rifts precisely because one core assumption held firm: allies do not turn their weapons or threats on each other.
Yet Donald Trump’s renewed rhetoric about annexing Greenland has forced policymakers, experts, and allies to confront a scenario that NATO has never had to face.
When Trump declared on Truth Social that “NATO becomes far more formidable and effective with Greenland in the hands of the UNITED STATES” and that “anything less than that is unacceptable,” it was not merely diplomatic bluster. It was a direct challenge to the sovereignty of Denmark, a NATO ally, and by extension to the very foundation of the alliance itself.
Greenland is an autonomous territory within the Kingdom of Denmark. Denmark is a NATO member. That simple fact turns Trump’s words from a bilateral dispute into an alliance-level crisis.
At the heart of NATO lies Article 5, the collective defence clause that states an attack on one member is an attack on all. It has been invoked only once, after the September 11 attacks in 2001. Crucially, it was designed to deter external enemies not to manage internal aggression.
NATO has weathered tensions between members before, but as experts point out, there is no precedent for an outright attack or coercive threat by one ally against another. Nicole Covey of the Canadian Global Affairs Institute notes that the alliance depends on a basic norm: allies do not attack each other if they want the alliance to survive.
If that norm collapses, the alliance itself may follow.
Gaëlle Rivard-Piché of the CDA Institute is even more blunt. If the United States were to attack a NATO ally, she argues, it would likely spell the end of NATO. Not because tanks would immediately roll, but because trust the glue that holds the alliance together would be irreparably shattered.
For Canada, the situation is deeply uncomfortable. Ottawa has rightly affirmed that Greenland’s future is a matter for Greenland and Denmark alone, and Prime Minister Mark Carney has emphasized that NATO already provides collective security for the territory.
This is the correct position legally, morally, and strategically. But it also highlights a long-standing vulnerability. Canada, like many allies, has relied heavily on U.S. leadership and spending to uphold NATO’s security guarantees. That dependence now looks risky.
As Rivard-Piché points out, years of underinvestment in defence have left allies scrambling to demonstrate credibility at a moment when unity is under stress. Canada’s pledge to move toward five per cent defence spending by 2035 is not just about meeting targets it is about ensuring Canada can meaningfully contribute to alliance security, especially in the Arctic.
Trump’s justification for his Greenland ambitions rests on claims of growing Russian and Chinese threats in the Arctic. Denmark disputes this narrative, arguing that the region is not “crawling” with hostile ships and that the security picture is being exaggerated.
That matters, because inflated threat perceptions can be used to justify drastic actions. While experts agree that a military confrontation in Greenland remains extremely unlikely, they also warn that it is no longer impossible. The Trump administration’s track record of unpredictability makes even remote risks worth taking seriously.
European allies are already responding by increasing their presence in the Arctic, walking a delicate line: supporting Denmark and Greenland while avoiding a direct confrontation with Washington. Their diplomatic “stick,” as Rivard-Piché puts it, is small especially when the U.S. still accounts for nearly half of NATO’s total defence spending.
This crisis is not just about Greenland. It is about whether NATO remains an alliance of equals bound by shared rules, or whether power alone determines outcomes.
If a dominant member can openly threaten the territory of another without consequences, Article 5 becomes hollow. Collective defence turns into selective defence. And smaller allies including Canada are left to wonder whether security guarantees apply only when it is convenient for Washington.
Rationally, a conflict between NATO allies should be unthinkable. Politically, it now feels disturbingly discussable.
The future of Greenland should indeed be decided by Greenlanders and Denmark nothing more, nothing less. But the way this moment is handled will echo far beyond the Arctic. It will signal whether NATO is still a rules-based alliance, or whether it is entering an era where even its own red lines can be tested.
If that line is crossed, NATO may not collapse overnight. But it would no longer be the alliance the world has relied on for the past 76 years.
