The US has long expressed interest in acquiring Greenland. Most recently, President Trump indicated a desire to ‘get’ the island by whatever means necessary. While the motivations remain somewhat unclear, there is – despite political rhetoric – no doubt as to Danish sovereignty over the island. Refusals to rule out the use of force could also be seen as a violation of the UN Charter. The current situation is peculiar, given the often-overlooked fact that the US has had extensive access to Greenland since 1941. A less-discussed but important question is whether this access is contingent upon US membership in NATO. Given President Trump’s past threats to withdraw from NATO, this post considers whether such a move would allow Denmark to withdraw from the existing defence agreement governing US military presence in Greenland. After setting out Denmark’s claim to sovereignty and the US acceptance thereof, the post examines the possibility that US withdrawal from NATO could trigger a fundamental change of circumstances requiring the US to give up its access to Greenland.
Sovereignty Over Greenland
Greenland’s connection to Denmark is long and complex. The first Norse settlements began around the year 985, led by Erik the Red, but these communities disappeared in the 15th century. A permanent European presence was re-established in 1721 when the missionary Hans Egede arrived from Norway. The current majority population – the Kalaallit, who are recognised as indigenous – are believed to have settled in Greenland during the 12th or 13th century as part of the Thule migration.
The link to Denmark is established via Norway, which since 1380 was in union with Denmark, and was later joined by Sweden under the Kalmar Union. Following the Napoleonic Wars, Norway was ceded to Sweden under the 1814 Treaty of Kiel, while Denmark retained Greenland, along with the Faroe Islands and Iceland.
Between 1915 and 1921, Danish diplomats secured international recognition of Danish sovereignty over Greenland. In the 1917 treaty on the sale of the Danish West Indies (now the US Virgin Islands), the US explicitly stated that it ‘will not object to the Danish Government extending their political and economic interests to the whole of Greenland’.
Following a dispute with Norway, the Permanent Court of International Justice confirmed in the East Greenland case that ‘Denmark possesses an old-established sovereignty over all Greenland’ (p 62). The last boundary issue involving Greenland was resolved peacefully with Canada in 2013.
US Recognition of Danish Sovereignty
The idea of acquiring Greenland first arose in 1867, shortly after the US purchased Alaska. It reappeared in 1910 as part of a proposed complex land swap (including the Philippines, West Indies, and Northern Schleswig) and again in 1917, when the US bought the West Indies. Another offer was made in 1946, when the US proposed $100 million in gold and a land exchange. Denmark rejected it, as it did a less formal approach by President Trump in 2019. These repeated offers to buy Greenland reflect an implicit recognition of Danish sovereignty.
Sovereignty was also affirmed in the 1941 defence agreement, signed with the Danish ambassador during the occupation of Denmark. Although the ambassador acted in a personal capacity, the agreement was ratified by the Danish government in 1945. The US’s primary aim was to prevent Germany from establishing a foothold in Greenland, which was valued for its role in weather forecasting, its strategic position as a potential refuelling and landing site for aircraft operating between the US and Europe, and as the only known source of cryolite – an essential mineral for aluminium production and thus critical to wartime aircraft manufacturing.
The 1941 agreement granted the US almost unfettered rights to establish military installations throughout Greenland. By the end of World War II, the US had exercised these rights at 17 locations, including major air bases at Narsarsuaq and Søndre Strømfjord.
This agreement affirmed Danish sovereignty, an affirmation repeated in 1951 when the defence agreement was renegotiated, aligning it with NATO’s collective defence framework. While the US retained significant rights, its activities were now limited to designated ‘defence areas’. As the 1951 agreement states: ‘Without prejudice to the sovereignty of the Kingdom of Denmark over such defense area and the natural right of the competent Danish authorities…’ (1951 Agreement, Article II(3)(b)).
The 1951 agreement, which remains in force, has been amended several times, most significantly in 2004. The 2004 Igaliku agreement formally limited US presence to Thule Air Base (now Pituffik Space Base), currently the only remaining US base in Greenland. From a peak of around 10,000 personnel, the US now maintains a contingent of approximately 150 on the base. The 2004 amendment also introduced formal consultation with the Greenlandic government, reflecting the growing political autonomy of the island’s indigenous population.
Until recently, the US had never questioned Danish sovereignty and had instead based its presence in Greenland first on the 1941 agreement and later on the 1951 agreement, the latter of which is clearly linked to NATO.
What if the US Leaves NATO?
The 1951 agreement is closely tied to NATO. The preamble and multiple provisions make clear that US access to Greenland is grounded in NATO’s collective defence structure. Article XIV(2) explicitly provides:
This Agreement, being in implementation of the North Atlantic Treaty, shall remain in effect for the duration of the North Atlantic Treaty.
While the 1951 agreement does not expressly state that US withdrawal from NATO would terminate it, the wording strongly implies that continued participation in the NATO framework is essential to its operation. This is significant, not only because President Trump has threatened to withdraw from NATO, but also because the agreement contains no other termination clause.
In 2023, the US Congress passed legislation requiring congressional assent for any US withdrawal from NATO. Even so, the procedure for withdrawal remains relatively straightforward, requiring only one year’s notice under Article 13 of the North Atlantic Treaty. As a matter of law, the NATO treaty would remain in force for the other 31 members. In practice, however, a US departure would profoundly affect the alliance.
Even so, the NATO Treaty would ‘remain in effect’, as stipulated by Article XIV(2). The only way for Denmark to exit the 1951 agreement may therefore be under the doctrine of rebus sic stantibus – that is, a fundamental change of circumstances.
Fundamental Change of Circumstances
Article 62(1) of the Vienna Convention on the Law of Treaties, which reflects customary international law, provides:
A fundamental change of circumstances which has occurred with regard to those existing at the time of the conclusion of a treaty, and which was not foreseen by the parties, may not be invoked as a ground for terminating or withdrawing from the treaty unless:
a) the existence of those circumstances constituted an essential basis of the consent of the parties to be bound by the treaty; and
b) the effect of the change is radically to transform the extent of obligations still to be performed under the treaty.
The threshold for invoking Article 62 is high. It has been consistently rejected by the International Court of Justice (ICJ) and has only been successfully invoked in the Racke case before the European Court of Justice. There, the European Community (now the EU) was permitted to terminate its 1976 Cooperation Agreement with the Socialist Federal Republic of Yugoslavia due to the outbreak of the Yugoslav wars and the disintegration of the Yugoslav state. A similar plea failed in the Gabčíkovo–Nagymaros case, where the ICJ rejected Hungary’s claim, despite the breakup of former Czechoslovakia.
In the Fisheries Jurisdiction case, the ICJ stated:
In order that a change of circumstances may give rise to a ground for invoking the termination of a treaty, it is also necessary that it should have resulted in a radical transformation of the extent of the obligations still to be performed. The change must have increased the burden of the obligations to the extent of rendering the performance something essentially different from that originally undertaken. (para 43)
The Danish motivation for the 1951 agreement was partly to secure recognition of Danish sovereignty and partly to secure collective self-defence. In fact, prior to the 1951 agreement, the Danish government specifically requested a security guarantee for ‘southern Denmark’ as compensation for a permanent US presence in Greenland (p 39). Thus, the object and purpose of the 1951 Defence Agreement is arguably to enable the use of Greenland for the collective defence of the North Atlantic area, in line with obligations under the NATO treaty.
A parallel may be drawn to the termination of the bilateral 1972 Anti-Ballistic Missile Treaty (ABM Treaty) between the US and the then Soviet Union. The ABM Treaty included a termination clause incorporating a variation of the rebus sic stantibus doctrine.
Article XV(2) of the ABM Treaty allowed either party to withdraw ‘if it decides that extraordinary events related to the subject matter of this Treaty have jeopardised its national interests’. In 2001, President Bush announced the US withdrawal from the ABM Treaty, arguing that the conditions affecting US national security had changed fundamentally and that the basis for the treaty no longer existed.
Determining whether a change is ‘fundamental’ is challenging, as the term lacks a clear definition and there is limited state and judicial practice. In the case of the ABM Treaty, President Bush argued that there was a fundamental change of circumstances because the Cold War had ended, the Soviet Union no longer existed, and the primary threat had shifted from superpowers to terrorists and rogue states. In Gabčíkovo–Nagymaros, the Court confirmed that a change in political circumstances is ‘certainly relevant’ (para 104).
There is consensus in the literature that a plea of fundamental change of circumstances may entitle a party to seek termination or suspension of a treaty (either by mutual consent, through an international tribunal) or by inviting the other party to negotiate its revision in good faith (p 866). Since the 1951 agreement lacks provisions for third-party dispute settlement, negotiation is arguably the only requirement.
Final Thoughts
As many aspects of the rebus sic stantibus doctrine remain underexplored, the legal implications of a US withdrawal from NATO are largely speculative. Nevertheless, given the 1951 agreement’s strong linkage to the NATO framework, such a withdrawal could offer Denmark (and Greenland) a plausible legal basis to seek termination of the agreement – and, by extension, US military rights in Greenland. But of course, the law is one thing, persuading the US to leave the island is another.