Donald Trump's repeated threats to seize control of Greenland are straining relations with US allies within Nato, prompting stark warnings that such an aggressive move could spell the end of the world's largest security alliance.
While historical tensions have occasionally flared between some Nato members, notably Greece and Turkey, the prospect of its most powerful country, the United States, annexing the territory of another ally would establish a dangerous precedent.
Mr Trump reaffirmed his stance on Sunday, declaring, "One way or the other, we're going to have Greenland." The White House has not ruled out the use of military force in this endeavour. Mr Trump stated his desire to prevent Russia or China from taking over the island, though he conceded that making a deal would be "easier." Greenland is a semi-autonomous island that forms part of the Danish realm.
Danish Prime Minister Mette Frederiksen has issued a severe caution, stating: "if the United States chooses to attack another NATO country militarily, then everything stops — including our Nato."
Decades of defending against outside threats
The North Atlantic Treaty Organization was formed by 12 nations in 1949 to counter the security threat posed by the Soviet Union during the Cold War. Nato's deterrence works through a strong American troop presence in Europe, with U.S. nuclear weapons stationed there.
Its ranks have grown since the founding Washington Treaty was signed to 32 countries after Sweden joined in 2024, worried by an increasingly aggressive Russia. Indeed, Nato officially considers its biggest threats to be Russia and international terrorism.
Nato's doors are open to any European country that wants to join and can meet the requirements and obligations. Importantly, Nato takes its decisions by consensus, so every member has a veto. The Trump administration has vetoed Ukraine's application.
Nato is built on the principle that an attack on any one of its ranks must be considered an attack on them all — the collective security guarantee enshrined in Article 5 of the treaty. The organisation's credibility depends on adversaries believing that all 32 allies will make good on that pledge.
It's a political commitment and not a legal obligation that can be enforced by any court. The only time it has ever been activated was in 2001, to support the United States in the wake of the 9/11 attacks on New York and Washington.
Article 5: collective defence against outsiders, not insiders
Trump said last year that he is committed to the Article 5 pledge, but he has also fuelled doubt about whether he would defend allies who don't spend enough on defence. However, his repeated threats against Greenland take things to a new level.
Article 5 would be moot in any U.S.-Denmark fight as there would be no unanimity to activate it. Besides diplomacy, Nato has no obvious way of dealing with open conflict among its members. Senior U.S. and Danish officials were due to hold talks on Wednesday.
Should things get worse, Denmark could trigger Article 4 of the treaty for official consultations if it feels that its sovereignty or territorial integrity is threatened. Article 4 talks do not automatically lead to any action.
A U.S. attack is almost certain to divide Nato. This happened when the United States led an attack on Iraq in 2003, with Britain and Spain backing while France and Germany led a group of others vehemently opposed.It's unclear which allies would side with Trump over Greenland.
An alliance led by the US
The United States is Nato's most powerful member. In real terms, it spends much more on defence than any other ally and far outweighs its partners in terms of military muscle. Washington has traditionally driven the agenda but has stepped back under Trump.
Nato is weaker without U.S. leadership, troops, equipment or other military assets. It's almost inconceivable that any ally would go to war with it, let alone hope to win.
Day-to-day work at the alliance's Brussels headquarters is led by former Dutch Prime Minister Mark Rutte.
As Nato's top civilian official, he chairs meetings of ambassadors in the North Atlantic Council most weeks. He chairs other "NACs" at ministerial level and summits of heads of state and government. Rutte runs Nato HQ, encourages consensus and speaks on behalf of all 32 members.
One of his main tasks is to ensure that the United States remains committed to Nato. As a result, he does not criticise Trump, and on Monday he sidestepped questions about Greenland and any Nato tensions over the island.
"All allies agree on the importance of the Arctic and Arctic security because we know that with sea lanes opening up there is a risk that the Russians and the Chinese will be more active," Rutte told reporters.
Asked whether Nato was in crisis over Greenland, Rutte said: "No, not at all."
Nato's military headquarters is based nearby in Mons, Belgium. It is always run by a top American officer. The current supreme allied commander, or SACEUR, is Air Force Lt. Gen. Alexus Grynkewich. Trump is his commander in chief.
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