Donald Trump’s expansionist aims on Greenland have opened a rift within NATO, especially after the US President threatened to invade the largest island in the world – part of Denmark – to gain control of it. In the last few hours, after a firm and compact European response (as not seen for some time), Trump has done an about-face, reassuring the other 31 members that the issue will be resolved with a diplomatic agreement.
But what would have happened if the US had attacked Greenland, the territory of another NATO country? This eventuality has never been taken into consideration and, in fact, within the North Atlantic Treaty of 1949 there is no article that regulates this option. NATO, in fact, is a mutual defense military alliance between states, designed with the aim of defending each other from external (and not internal) attacks.
In the event of a US attack on Greenland, the credibility of the Atlantic Pact would be undermined, with the risk of a huge internal crisis within NATO, an inconvenient option not only for Europe – which would lose its alliance with the strongest military power in the world – but also for the USA, which would have to give up all the NATO military bases in the Old Continent, which are essential for maintaining their military supremacy. Suffice it to say that, in Italy alone, there are 120 American bases, including Sigonella or Camp Darby.
What does the article say? 5 of the Atlantic Pact in the event of an attack against a NATO country
The idea of a NATO country attacking another member state of the Alliance is considered so remote that Article 5 of the Atlantic Pact – considered the pillar of this defensive military alliance – does not provide for this eventuality.
The article, in fact, establishes:
The parties agree that an armed attack against one or more of them in Europe or North America will be considered as a direct attack against all the parties, and consequently agree that if such an attack occurs, each of them, in the exercise of the right of self-defense, individual or collective, recognized by the art. 51 of the Charter of the United Nations, shall assist the party or parties so attacked by taking forthwith, individually and in concert with the other parties, such action as it deems necessary, including the use of armed force, to re-establish and maintain security in the North Atlantic region (…)
If a state were to attack one or more NATO members, that action would therefore be considered as a direct attack against all the countries of the Alliance, which would therefore be called upon to intervene with a collective defense. However, it is not made clear in any way what would happen if two NATO members went to war with each other.
It would in fact be a legal paradox for the Alliance, given the nature of the Atlantic Alliance: NATO was founded in 1949 with the aim of guaranteeing the mutual protection of the States Parties in Europe and North America. Back then, the main threat was represented by the Soviet Union: after its dissolution in the early 1990s, NATO “reinvented” itself with the main aim of guaranteeing world peace and security.
Likewise, in the event of a mutual attack, states could have invoked Article 4, which provides:
The Parties will consult each other whenever, in the opinion of either of them, the territorial integrity, political independence or security of any of the Parties is threatened.
Even in this case, the Pact does not establish the methods of action in the event that a NATO member threatens the territorial integrity of another party, as happened with the USA.
Nonetheless, the fact that the North Atlantic Treaty does not contemplate this hypothesis does not 100% exclude that it could occur: therefore, yes, a NATO member could attack its ally. It all depends on the strategy that each state decides to adopt and the advantages – in economic, political and international prestige terms – that it can obtain.
Would Europe respond to a Trump US attack on Greenland?
As several international relations experts point out, if Trump had indeed attacked Greenland, it is unlikely that Denmark or other European NATO members would decide to intervene in defense of the island. Simply because it does not have the military numbers to compete with the USA: the United States, in fact, has over 1.3 million active military personnel, divided across all its armed forces; Denmark has around 13,100.
Added to this is the fact that, according to official NATO data, in 2025 Washington spent around 845 billion dollars on defense, while all the other 31 member states spent a total of 559 billion dollars. It should also be remembered that the USA, unlike Copenhagen, already has a military base in Greenland, the Pituffik Space Base, from which it would have been much simpler to coordinate all operations.
In short, the US invasion of Greenland would have mainly caused a weakening of the Atlantic Alliance, with the risk that NATO would end up crumbling internally, changing the global geopolitical balance after 76 years of coalition.
