Denmark, Greenland, and the United States: Whether a U.S. Purchase of Greenland is Legal

During his first term, President Donald Trump expressed interest in purchasing Greenland, a semi-autonomous country of the Kingdom of Denmark, which comprises three constituent countries that are Denmark, the Faroe Islands, and Greenland. He cited security-based and resource-based rationales. Greenland’s location in the North Atlantic could provide strategic advantages against Russia and China, and its vast reserves of natural resources could be used for industrial and military purposes. Since Greenland is a constituent country of the Kingdom of Denmark, it can maintain its own self-governing authority but lacks full sovereignty over foreign affairs and defense and must refer to Denmark when making decisions involving those matters. In response to President Trump’s statements, Denmark and Greenland were firmly opposed to his pursuit. Ultimately, no deal was obtained between Denmark and the U.S. and Greenland remained under the Kingdom of Denmark. Now, in his second term, President Trump has revived his efforts in purchasing Greenland, citing the same reasons. Yet revitalized interests are far from enough for President Trump to acquire Greenland. Under current international and domestic law and legal precedent, the United States’ acquisition of Greenland would be an illegal act.

The Kingdom of Denmark and the United States are members of the United Nations (UN) and are subject to the jurisdiction of the International Court of Justice (ICJ). As members of the UN, both countries must adhere to the international laws such as the Vienna Convention on the Law of Treaties (VCLT). In international law, one way to obtain sovereignty over a territory from another state is cession. Cession involves the sovereignty of a territory being ceded over from one state to another state under mutual consent that is often expressed through a treaty. The requirements for a treaty are outlined in the VCLT which dictate that both states must consent to the agreement otherwise known as “consent to be bound”. This consent to be bound can be recognized by various methods such as signature, ratification, accession, or exchange of instruments that compose the treaty. Through this consent to be bound, the parties involved are agreeing to follow the provisions of the treaty. Treaties that address cession of a territory would only be valid under international law if all parties involved agree to the provisions of that treaty. An example of cession through treaty in history is the Kingdom of Denmark’s cession of the Danish West Indies, which includes St. Thomas, St. John, and St. Croix, to the U.S. for $25 million in gold at the time. This treaty is valid because both parties ratified it, meaning that consent to be bound was present. However, the VCLT prohibits countries from using threat or force to obtain a treaty under Article 52, meaning a treaty may be voided if it was procured through coercion or threat. A major example of coercion or threat is military coercion. In regards to Greenland, current U.S. plans to increase military presence in Greenland can be interpreted as threatening and coercive, so any treaty of cession obtained under these conditions can be voided. 

Currently, the U.S., Greenland, and Denmark already have a treaty called the Defense of Greenland Agreement which authorizes U.S. military presence in Greenland. Although the U.S. only has one military base in Greenland, Article 1 of the Defense of Greenland Agreement references future bases. This reference implies that the U.S. can establish military bases in Greenland but cannot arbitrarily do so because the Defense of Greenland Agreement was negotiated for mutual aid purposes. Article 60 of the VCLT emphasizes that a treaty is voidable if one party breaches provisions in the treaty. Increased U.S. military presence authorized for collective defense as leverage for territorial acquisitions defies this mutual aid purpose of the Defense of Greenland Agreement if military presence only serves U.S. interests while ignoring Greenlandic and Danish interests. In breaching the provision on mutual aid, Greenland and Denmark are allowed to revoke the Defense of Greenland Agreement and stop the U.S. from maintaining any military presence in Greenland. Heightened U.S. military presence also violates Article 26 of the VCLT which requires treaties be administered with good faith because increasing U.S. military presence for U.S.-centric purposes does not satisfy good faith. Another treaty that is still legally binding between Greenland, Denmark, and the U.S. is the treaty that ceded the Danish West Indies to the U.S. Because that treaty contains a declaration that the U.S. will not hinder the Danish government from extending its interests to the entirety of Greenland and recognizes Danish sovereignty over Greenland, the U.S. still has to respect those provisions of that treaty. Although this treaty was ratified before the VCLT and the articles of the VCLT do not apply to it, the treaty has never been nullified and is therefore still enforceable. 

The existence of a U.S. military base in Greenland is also not enough to justify U.S. sovereignty in Greenland. The U.S. is obligated to cooperate with Greenland and Denmark as noted in Article 3 of the Defense of Greenland Act. By agreeing to cooperate with Greenland and Denmark, the U.S. affirms that it does not have sole control over its military bases in Greenland and that it must consult Denmark and Greenland when making changes to its military operations in Greenland. As a general rule, the U.S. also acknowledges that military bases are not owned by the U.S. and remain under the sovereignty of the host country. This means that the U.S. does not automatically have sovereignty regarding Pituffik Space Base nor can it act independently regarding military operations on its bases in Greenland. 

As for domestic law within Denmark, Greenland, and the U.S., all nations have their own rules laying out foreign affairs and territory acquisition/relinquishment. In Denmark and Greenland, the Act on Greenland Self-Government alongside Denmark’s Constitution acts as the constitution for Greenland. The Act on Greenland Self-Government emphasizes Greenland’s right to self-determination and self-government in Chapter 1. In Chapter 4, Greenland’s Parliament is allowed to engage in affairs alongside Denmark, and Denmark’s power and responsibility is not limited to only Denmark in cases involving foreign and security policy for the entire Kingdom of Denmark. Greenland is also allowed to engage in international affairs that pertain only to Greenland as long as Denmark is informed on any affairs. Conversely, Denmark will notify Greenland of any agreements, negotiations, and terminations that involve Greenland, allowing Greenland to provide remarks on such engagements. That means in foreign affairs involving Greenland, Denmark must have approval from both Greenlandic and Danish governments. Furthermore, because cession of Greenland encompasses Greenland’s independence, there are additional requirements needed to fulfill U.S. possession of Greenland.

Another critical limitation on territorial cession is the principle of self-determination, which is recognized under international law and affirmed in the Western Sahara Advisory Opinion where the ICJ held that the will of the people of a territory must be taken into account when determining sovereignty. This is the additional requirement needed to carry a U.S. acquisition of Greenland. Because Greenland is a self-governing territory with its own people and political institutions, any transfer of sovereignty without the clear and democratic consent of the Greenlandic population would violate international law. This is highlighted in the Act on Greenland Self-Government where Chapter 8 stresses the importance of holding a referendum for Greenlandic independence with the consent of the Danish Parliament. Because transfer of sovereignty of Greenland to the U.S. involves Greenland’s independence and Denmark’s sovereignty of Greenland, a referendum would need to be held. Applied to a potential U.S. purchase of Greenland, this means that approval is required not only from the legislative and executive authorities of both Denmark and Greenland, but also from the Greenlandic population itself through a majority-supported referendum. If any one of the Danish government, Greenlandic government, or Greenlandic population fails to agree to a U.S. acquisition of Greenland, then the deal would fail. Therefore, even if Denmark wanted to give Greenland to the U.S., such an action would be considered illegal without a referendum or equivalent expression of popular consent by the Greenlandic people. This is also strengthened by the ICJ’s ruling in East Timor (Portugal v. Australia) in 1991 where the ICJ recognized the right to self-determination as an obligation to the international community as a whole. In that case, although the ICJ ultimately declined jurisdiction, it affirmed that the people of East Timor had a legal right to determine their own political status, regardless of agreements made by other states. Applying the Western Sahara Advisory Opinion and East Timor case to Greenland means that even if Denmark and the United States reached a bilateral treaty transferring sovereignty of Greenland, such an agreement would be invalid if it does not reflect the free and popular will of the Greenlandic people.

Meanwhile for the U.S., under the Constitution, President Trump would need authorization from Congress. Under Article II, Section 2 of the U.S. Constitution, treaties cannot be made by the President without two-thirds Senate approval. A purchase of Greenland would most likely be done through a treaty with Denmark, meaning it would be illegal for President Trump to purchase Greenland without the Senate approval. Although an executive order could be utilized, instances of when executive orders succeeded in acquiring territory still required Congressional approval through joint resolutions with simple majority votes from both houses of Congress. Examples of joint resolutions being used to acquire territory for the U.S. include the Newlands Resolution that annexed Hawaii and the joint resolution that annexed Texas. These examples imply that the U.S. could annex Greenland with Presidential and simple majority Congressional approval; however Texas agreed to being annexed and Hawaii subsequently agreed to statehood in 1959, denoting that some level of consent was obtained. Though the legitimacy of Hawaii’s annexation remains historically contested, the use of a joint resolution rather than a treaty set a procedural precedent for territorial acquisition. A case where the U.S. did renounce sovereignty of a territory when the majority of citizens of that territory wanted independence is the Philippines through the Treaty of Manila, showing that self-determination is upheld in U.S. foreign affairs. These cases indicate that the U.S. would still need to obtain consent from Greenland if Greenland were to be annexed and that Greenland could declare independence through a referendum and treaty. 

The proposed purchase of Greenland by the United States is heavily constrained by clear legal limits under both international and domestic law. While territorial acquisition through cession has historical precedent, contemporary international law imposes far stricter requirements, particularly emphasizing consent, the prohibition of coercion, and the right to self-determination. In addition, treaty law which is guided by frameworks such as the VCLT renders any agreement invalid if it is achieved through coercion, further limiting the legality of any pressured transaction. Beyond international law, both Danish and U.S. domestic legal systems and governments impose procedural and constitutional barriers that make transfer of territory highly unlikely without broad political consent. Taken together, these legal principles and precedents demonstrate that, under current law, a U.S. acquisition of Greenland would be impossible with the rules governing sovereignty and territorial integrity in the modern international system. The Greenland question ultimately illustrates a broader transformation in international law where territory is no longer treated as a transferable commodity between states. Instead, a territory holds its own legal right in determining its future. This shift explains why actions that were once legally permissible, such as the U.S. purchase of Danish West Indies or annexation of Texas and Hawaii, would face overwhelming legal barriers today. In this regard, the illegality of a U.S. acquisition of Greenland showcases a deeper legal evolution designed to prevent unilateral or coercive territorial expansion.

Edited by Uma Rajan.

This piece was reviewed and finalized by Gabi Fabozzi, Qizhen (Kiara) Ba, and Jasmine Lianalyn Rocha.