Legal Obstacles to U.S. Acquisition of Greenland

In early 2026, President Donald Trump renewed a goal he had set in his first term, acquiring Greenland, a territory over which Denmark exercises sovereignty. Trump escalated the situation by threatening to use force to do so, warning that he was “going to do something on Greenland, whether they like it or not,” and that while he “would like to make a deal the easy way, … if we don’t do it the easy way we’re going to do it the hard way.” He later told reporters aboard Air Force One, “one way or the other, we’re going to have Greenland.” The president’s threats have also taken the form of Truth Social posts showing him planting the U.S. flag on Greenland and chairing a meeting with European leaders, with a map in the background showing Canada and Greenland covered in an American flag. Taking a less bellicose stance, Secretary of State Marco Rubio has informed members of Congress that the United States is trying to buy Greenland, although he, too, has at times left the door open to using force.

As we’ll explain below, the acquisition of territory by the threat or use of force is not only prohibited under international law binding on the United States, but that rule forms the backbone of the entire international system and constitutes the major achievement of the post-WWII era. While sometimes violated, to be sure, naked threats to acquire territory by threat or use of force without even purporting to justify that action under international law (say, as an act of self-defense) are rightfully seen as belonging to a bygone era that was less secure and less prosperous than one in which sovereignty and stability are valued over sheer military might – and perhaps most important, in which self-determination is recognized as a foundational right, while imperial conquest is outlawed.   

Danish Prime Minister Frederiksen has responded to the threats by unambiguously stating, “Greenland is not for sale. Greenland is not Danish. Greenland belongs to Greenland.” Other leaders have echoed her stance. For instance, seven European heads of State issued a joint statement on Greenland that noted “Greenland belongs to its people. It is for Denmark and Greenland, and them only, to decide on matters concerning Denmark and Greenland.” They emphasized that all NATO Allies were obligated to “uphold the principles of the UN Charter, including sovereignty, territorial integrity and the inviolability of borders. These are universal principles, and we will not stop defending them.” European Union leaders likewise pointed to “territorial integrity and sovereignty,” confirming the “EU stands in full sovereignty with Denmark and the people of Greenland.” In the hope of deterring a U.S. attack, a number of European States also deployed small troop contingents to Greenland as a military “trip wire” (although framed as an “exercise” to avoid unduly escalating the situation), one that would presumably draw those countries into any conflict between Denmark and the United States. 

Whether because the world rallied around the bedrock principles of international law in response to Trump’s threats or for other reasons, on Jan. 21 at Davos, the president shifted towards appearing to rule out the use of force – “I don’t have to use force. I don’t want to use force. I won’t use force” – while still insisting that the United States must acquire the territory, stating:

We want a piece of ice for world protection, and they won’t give it. We’ve never asked for anything else … So, they have a choice. You can say yes and we will be very appreciative, or you can say no and we will remember.

Following a subsequent meeting with NATO Secretary General Rutte, Trump announced on Truth Social that they “formed the framework of a future deal with respect to Greenland and, in fact, the entire Arctic Region,” one that, “if consummated, will be a great one for the United States of America, and all NATO Nations.” 

At this point, it is unclear whether Trump is willing to shelve his desire to acquire Greenland, including the use of force to achieve that objective, if he finds the European response unsatisfactory. The track record thus far demonstrates that his hunger for Greenland is something he has difficulty shaking. Moreover, as Trump’s positions frequently vacillate, little is certain at this point. The risk of cleaving the transatlantic alliance, which has benefitted the Western world (and globally, all who value global goods like open shipping lanes) for over 75 years, remains real.  

Trump’s persistent desire to acquire Greenland raises the question of how international law treats the acquisition of another State’s territory, whether by force or by other means. There are two general methods. The first is “conquest,” which the Permanent Court of International Justice defined in 1933 as “a cause of loss of sovereignty when there is war between two States and by reason of the defeat of one of them sovereignty over territory passes from the loser to the victorious State” (Eastern Greenland case, page 47). Note that war, or “international armed conflict” as it is labeled today, can be triggered even by an unopposed occupation of the territory of one State by another (Common Article 2, Geneva Conventions I-IV), as could be the case if U.S. troops seized a remote area of Greenland. The second is “cession,” which denotes the consensual transfer of territory from one State to another. Historical U.S. examples include the purchase of Louisiana from France in 1803 or Alaska from Russia in 1867.

In this essay, we examine these modalities of territorial acquisition in general and with respect to Greenland. Before doing so, however, it is essential to emphasize that Denmark unquestionably exercises sovereignty over Greenland. As one of us (Mike) explained in a previous Just Security article, Greenland

enjoys broad autonomy under Denmark’s 2009 Self-Governing Act, especially in internal affairs. However, that autonomy is an issue of domestic law. Under international law, Denmark undeniably enjoys sovereignty over Greenland. In 1933, the Permanent Court of Justice, in a dispute between Norway and Denmark, found that Greenland was a Danish possession (Legal Status of Eastern Greenland). Subsequently, Denmark listed Greenland as a non-self-governing territory under Chapter XI of the UN Charter. In 1953, Greenland was incorporated into Denmark through domestic legislation, a status recognized by the General Assembly the following year (GA Res. 849). The United States has long acknowledged Danish sovereignty over Greenland (see, e.g., the 1916 Secretary of State declaration, the 1951 Defense of Greenland Agreement, and the 2004 Amendment to the Defense of Greenland Agreement). 

Therefore, military action into Greenland would be a use of force against Denmark, which would destroy the NATO alliance from within. Cessation would also be a matter to be negotiated at the inter-State level between Denmark and the United States, although, as will be explained, the people of Greenland would have a say in any such negotiations.

Conquest and its Prohibition 

Although international law historically recognized that title to territory could be transferred through conquest, as we referenced above, that premise was put to rest in the 20th century (see here). Most notably, Article 2(4) of the UN Charter addresses the issue head-on: 

“All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the purposes of the United Nations” (emphasis added). 

It is a prohibition that undeniably reflects customary international law (Paramilitary Activities, ¶ 190). 

A U.S. military operation, even if unopposed, to seize Greenland would qualify as a violation on both grounds. First, the mere presence of U.S. troops (or even operations into Greenland from outside the territory, such as missile strikes) would amount to a violation of Danish territoriality. Second, the annexation of Greenland as a result of a U.S. use of force (or any alteration in the territory’s governance) would violate 2(4) on the basis that the force was directed at Denmark’s political independence.

During the decolonization that followed the immediate post-WWII period, the international community reaffirmed this prohibition in several forms. In 1970, the General Assembly, in its Declaration on Friendly Relations (adopted by consensus), famously confirmed the prohibition: “The territory of a State shall not be the object of acquisition by another State resulting from the threat or use of force” (GA Res. 2625(XXV)). Along the same lines, the 1974 Definition of Aggression Resolution states that “no territorial acquisition or special advantage resulting from aggression is or shall be recognized as lawful” (art. 5(3)). And in its 1986 Frontier Dispute judgment, the International Court of Justice (ICJ) emphasized that even where States have effective control over territory, they do not acquire title if that control was secured through unlawful means (¶ 63).

Since any forcible action against Greenland would trigger an international armed conflict between the United States and Greenland, the prohibition on the permanent transfer of territory during occupation under the law of armed conflict is also relevant. Specifically, Article 47 of Geneva Convention IV provides that the rights of protected persons in occupied territory are inviolable. It is a fundamental LOAC rule that occupation of territory is temporary and is not permitted to morph into annexation. Another way of stating this prohibition is that the rights of protected persons in occupied territory would be violated if title to that territory changed hands. The ICRC’s Commentary to Article 47 explains,

The law of occupation is predicated on the temporary and provisional nature of occupation; it thus follows that the Occupying Power does not acquire sovereign rights over the occupied territory. 

Although international humanitarian law does not expressly prohibit the forcible acquisition of territory, any such annexation during an ongoing occupation would inescapably lead to violations of that law. For example, the annexed territory would, by definition, be subject to the legislation of the annexing State, which would exceed the limited scope of the Occupying Power’s legislative competences in the occupied territory. More broadly, unilateral annexation would entail the exercise of sovereignty over the occupied territory and purport to create a permanent situation that would eliminate the status quo ante. In the ICRC’s view, both of those consequences are inconsistent with the law of occupation’s underlying principles, namely that the Occupying Power does not acquire any sovereignty over the occupied territory and that it must maintain the status quo ante. 

The U.S. Department of Defense (DoD) Law of War Manual makes the same point: “Protected persons who are in occupied territory shall not be deprived, in any case or in any manner whatsoever, of the benefits of the GC by any change introduced…by any annexation…of the whole or part of the occupied territory” (§ 11.6.3). 

The prohibition on acquiring territory through conquest is likewise supported in case law. Perhaps most notably, in United States v. Ulrich Griefelt et al. (the RuSHA case), the U.S. Military Tribunal at Nuremberg held that 

Any purported annexation of territories of a foreign nation, occurring during the time of war and while opposing armies were still in the field, we hold to be invalid and ineffective. Such territory never became a part of the Reich but merely remained under German military control by virtue of belligerent occupancy. 

Treaties to which the United States is not Party, but that represent further testament to the extent to which international law condemns the acquisition of territory by conquest, are in accord. The 1977 Additional Protocol to the Geneva Conventions expressly provides that “occupation of a territory… shall [not] affect the legal status of the territory in question” (art. 4). Moreover, under international criminal law, the Rome Statute of the International Criminal Court criminalizes “annexation by the use of force of the territory of another State or part thereof” (art. 8bis(2)(a)).

The Duty of Non-Recognition of Annexed Territory

Importantly, even if the United States seized Greenland and annexed it, other States would be prohibited from treating the territory as if it were the United States. As noted in the Declaration on Friendly Relations, “No territorial acquisition resulting from the threat or use of force shall be recognized as legal.” In its Wall advisory opinion, the ICJ recognized this statement as reflecting customary international law (¶ 87, implemented at ¶ 159). 

In Paramilitary Activities, the ICJ noted that the United States has long recognized the principle of non-recognition of territory unlawfully seized. In doing so, it pointed to the 1933 Montevideo Convention on the Rights and Duties of States (¶ 189). Article 11 provides, 

The contracting states definitely establish as the rule of their conduct the precise obligation not to recognize territorial acquisitions or special advantages which have been obtained by force whether this consists in the employment of arms, in threatening diplomatic representations, or in any other effective coercive measure. The territory of a state is inviolable and may not be the object of military occupation nor of other measures of force imposed by another state directly or indirectly or for any motive whatever even temporarily.

As the United States is Party to the Montevideo Convention, other Parties (States in Central and South America) would be prohibited by the terms of the treaty from recognizing the unlawful U.S. acquisition of Greenland. More significantly, as noted by the ICJ, Article 11 reflects the customary law principle of non-recognition, which would bind all States from recognizing the U.S. action. 

UN organs, with the support of the United States, have likewise emphasized the obligation not to recognize unlawful seizure of territory. For instance, in 1990, following Iraq’s annexation of occupied Kuwait, the Security Council “call[ed] upon all States, international organizations and specialized agencies not to recognize that annexation and to refrain from any action or dealing that might be interpreted as an indirect recognition of the annexation” (SC Res. 662). Following Russia’s unlawful annexation of Crimea in 2014 following its occupation and a sham referendum, the General Assembly did the same, “Calling upon all States, international organizations and specialized agencies not to recognize any alteration of the status of the Autonomous Republic of Crimea and the city of Sevastopol on the basis of the above-mentioned referendum and to refrain from any action or dealing that might be interpreted as recognizing any such altered status” (GA Res. 68/262).

What about Self-Defense?

Might the seizure of Greenland be justified on U.S. security grounds? After all, administration officials have suggested that taking control of Greenland is necessary in the face of threats by Russia and China in the Arctic (we must note here that working within NATO to address any security concerns would be a lawful, legitimate, and likely effective course). While self-defense is a “circumstance precluding the wrongfulness” of a use of force under Article 51 of the UN Charter (Articles on State Responsibility, art. 21), forcible defensive actions may only be directed at a State that has engaged in an “armed attack” against the State taking them. Accordingly, even if U.S. security were threatened by Russia or China moving into the Arctic, that fact would not, in itself, legally justify any action against Denmark. 

It should not be lost in the current discussion that Russian President Vladimir Putin attempted to employ pretextual self-defense claims in his war of attempted conquest against Ukraine. That attempt remains ongoing but has been resoundingly and repeatedly rejected by the United States, the United Nations General Assembly (but not the Security Council due to Russia’s veto), the European Union, notable international law associations, and scholars. Russia’s purported annexation of Crimea (undertaken largely without firing shots in 2014) and parts of Ukraine’s Donbas region are not recognized by the majority of States, owing to their obligation not to do so as described above. 

Solely for the sake of discussion, let us assume that a State had a valid self-defense claim, was involved in an international armed conflict as a result of an armed attack against it, and attempted to lay claim to territory it seized during that conflict. This raises the question of whether a State legitimately acting in self-defense may acquire territory. In this regard, the International Law Commission (ILC) has concluded, correctly in our view, that “States may not recognize as lawful, for example, a unilateral acquisition of territory procured by the use of force, even if the use of force was arguably lawful” (Third Report on State Responsibility, page 107). This conclusion also bears on the earlier point regarding other States’ recognition of territory acquired through conquest.

Finally, U.S. threats to acquire Greenland by conquest already constitute a violation of international law. As we explained in an earlier Just Security article, “Article 2(4) of the UN Charter and customary law prohibit States from even threatening to use force unlawfully if the threats are communicated to the threatened State, coercive, and capable of being carried out.” Administration officials have repeatedly indicated a willingness to resort to force if Denmark does not cede Greenland to the United States (though they have see-sawed on these threats in recent days, it is difficult to discount them once issued so repeatedly and forcefully by a sitting head of State); the coercive nature of the threats is evidenced by the alarm they have caused in Europe, Canada, and beyond; and the U.S. military has recently demonstrated its substantial capacity to employ significant force abroad. Perhaps most notably, the U.S. willingness to attack Venezuela in clear violation of international law renders the threats against Greenland much more credible than they would otherwise be.

Cession: When it is Permissible Under International Law

While international law prohibits the acquisition of territory through force, it does not prohibit territorial change altogether. Instead, a sovereign may transfer parts of its territory by cession, usually through a treaty or other international agreement. When that happens, the transferring State relinquishes title, and the acquiring State assumes sovereignty. The practice is quite rare in the contemporary (post-WWII) era and is generally limited to minor border adjustments or resolving longstanding ambiguities in inherited post-colonial borders. Such exchanges typically result in compensation or form part of a broader political agreement. 

Contemporary international law subjects such transfers to two conditions. First, the State making the transfer must give its consent freely; it cannot be coerced. Second, the transfer must be consistent with the right of self-determination under international human rights law. 

Lack of coercion: Since any transfer would likely be concluded by treaty, the Vienna Convention on the Law of Treaties establishes the framework for determining the validity of consent to transfer. Article 52 of the instrument provides, “A treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the Charter of the United Nations.” 

That is precisely what has happened in the case of U.S. demands for Greenland. It is almost unimaginable that the threat of U.S. military action would not have significantly influenced any agreement by Denmark to transfer Greenland, or portions of the territory, to the United States. Although the United States is not a Party to the Vienna Convention, Article 52 is well recognized as reflecting customary law (Fisheries Jurisdiction, ¶ 24).

Self-determination: The second condition is that a transfer be consistent with the principle of self-determination. Article 1(2) of the UN Charter recognizes the principle, which also plays a prominent role in the International Covenant on Civil and Political Rights (ICCPR) and the International Covenant on Economic, Social and Cultural Rights (ICESCR). They provide, in identical Articles 1(1), “All peoples have the right of self-determination. By virtue of that right, they freely determine their political status and freely pursue their economic, social and cultural development.” The United States is Party to the ICCPR (but not the ICESCR).

The ICJ has applied the principle of self-determination in the context of territory on multiple occasions. In its Western Sahara advisory opinion, the Court “confirm[ed] and emphasize[d] that the application of the right of self-determination requires a free and genuine expression of the will of the peoples concerned” (¶ 54). Similarly, in its East Timor judgment, the Court labeled the assertion that the right is of erga omnes character (owed to all States) as “irreproachable,” and called self-determination “one of the essential principles of contemporary international law” (¶ 29). 

Denial of a right to participate in the decision to transfer territory is a paradigmatic example of a violation of the right to self-determination. It is a right that applies to “peoples.” That term is not well-defined in international law. However, as a general matter, it refers to a significantly large group of individuals who share common characteristics such as history, language, ethnicity, or religion; have a connection to the territory concerned; and consider themselves a distinct community.

Despite the imprecision of the term “peoples,” it is clear that it applies to Greenland’s inhabitants. Indeed, the 2009 Act on Greenland Self-Government expressly recognizes that “the people of Greenland is a people pursuant to international law with the right of self-determination” (pmbl). Importantly, this is a legally binding acknowledgment that the people of Greenland have a right to participate in determining their political status. Moreover, the act provides, “Decisions regarding Greenland’s independence shall be taken by the people of Greenland ((art. 21(1)). If they enjoy a legal right under Danish law to decide on independence, they surely enjoy no lesser right vis-à-vis the transfer of sovereignty.

Moreover, as noted, Greenland was formerly a non-self-governing territory and has been moving towards independence. As such, it is subject to the decolonization principles outlined in the 1960 UN General Assembly Resolution 1541(XV). Those principles emphasize the “freely expressed wishes” of a territory’s people concerning the status of the territory upon which they reside. In its Chagos advisory opinion, the ICJ observed that although the resolution was formally a recommendation, “it has a declaratory character with regard to the right to self-determination as a customary norm” (¶ 152). That means under international law, the principle of self-determination prohibits the transfer of sovereignty over a territory inhabited by a “people” without their consent.

Finally, the Declaration on Friendly Relations repeatedly highlights the right of self-determination. It provides,

By virtue of the principle of equal rights and self-determination of peoples enshrined in the Charter of the United Nations, all peoples have the right freely to determine, without external interference, their political status and to pursue their economic, social and cultural development, and every State has the duty to respect this right in accordance with the provisions of the Charter.

The establishment of a sovereign and independent State, the free association or integration with an independent State or the emergence into any other political status freely determined by a people constitute modes of implementing the right of self-determination by that people.

The Declaration then ties together the right of self-determination and the use of force: “Every State has the duty to refrain from any forcible action which deprives peoples referred to in the elaboration of the principle of equal rights and self-determination of their right to self-determination and freedom and independence.” The principle would apply equally to the threat of the use of force that might, in turn, lead to non-consensual cession. 

As discussed above with respect to territory secured by force, other States would be barred from recognizing the transfer of Greenland through cession, given that it was secured by coercion and failed to respect the right of the Greenlandic people to self-determination. The ILC has acknowledged as much in Article 41(2) of its Articles on State Responsibility: “No State shall recognize as lawful a situation created by a serious breach within the meaning of article 40, nor render aid or assistance in maintaining that situation.”

As a consequence of the above, any transfer of Greenland to the United States would require the freely given consent of both Denmark and Greenland. This is so under both domestic Danish law and international law. That consent is unlikely.

Finally, if Denmark were to cede Greenland to the United States in violation of the right of self-determination of the Greenlandic people, under principles of the law of State responsibility, the United States arguably would also shoulder responsibility for that “internationally wrongful act.” Article 18 of the Articles on State Responsibility, which in significant part reflects customary international law binding on the United States, provides that “a State which coerces another State to commit an act is internationally responsible for that act if: (a) the act would, but for the coercion, be an internationally wrongful act of the coerced state; and (b) the coercing State does so with the knowledge of the circumstances of the act.” Threats of the use of force clearly qualify as coercive (“conduct which forces the will of the coerced State”) according to the Article’s commentary. Moreover, as the right of self- determination is customary, all States shoulder the obligation. And, of course, the United States is aware of the circumstances, for it is their author.

Concluding Thoughts

It is indisputable that, under current circumstances, any acquisition of Greenland by the United States–whether by force or through coerced agreement–would directly violate fundamental principles of international law, including the prohibition on the threat or use of force, the principle of non-recognition of territorial acquisition by such means, the invalidity of agreements secured through coercion, and the right of the people of Greenland to self-determination. These are insurmountable legal barriers to the legitimacy of such an action. Accordingly, consistent with long-standing legal norms, any U.S. effort to claim sovereignty over Greenland, or parts of the territory, must continue to be met with international non-recognition and political condemnation.

FEATURED IMAGE: A smartphone displays a post by U.S. President Donald Trump on the Truth Social platform showing a composite image featuring Trump alongside U.S. Secretary of State Marco Rubio and U.S. Vice President J.D. Vance, with the U.S. flag and a sign reading Greenland, U.S. Territory Est. 2026, while a map of Greenland is visible in the background, on January 20, 2026 (Photo illustration by Cheng Xin/Getty Images)