The Trump Administration’s sabre-rattling over securing control of Greenland, the sovereign territory of Denmark, continues to escalate. The President’s apparent concerns with Greenland date back to his first term, when he asked his team to explore purchasing Greenland for its “valuable resources” and tasked the National Security Council with assessing options for acquiring the territory. But in this term, the Administration is publicly discussing seizing the territory by military force, and U.S. allies and members of Congress are taking these developments seriously.
Just last week, Deputy Chief of Staff Stephen Miller warned, “Nobody’s going to fight the United States militarily over the future of Greenland.” In justification, he claimed, “For the United States to secure the Arctic region, to protect and defend NATO and NATO interests, obviously, Greenland should be part of the United States.” Less bombastically, Secretary of State Marco Rubio told members of Congress that the United States is attempting to purchase Greenland. Yet he caveated that desire by noting to reporters, “If the president identifies a threat to national security of the United States, every president retains the option to use military force.”
And on Wednesday, President Trump told the New York Times the only limit to his international actions was “My own morality. My own mind. It’s the only thing that can stop me…. I don’t need international law.” Two days later, Trump appeared to apply that approach to Denmark, warning 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, but if we don’t do it the easy way we’re going to do it the hard way.” As to justification, Trump warned, “If we don’t do it, Russia or China will take over Greenland.” However, it was unclear why current or expanded agreements with NATO Ally Denmark, such as the Defence Cooperation Agreement finalized last June, and the renewed commitment by other States to enhancing Arctic security (e.g., here and here), would not suffice to secure the strategically important territory.
These threats have led some to wonder whether NATO would have to defend Denmark if the United States attempted to seize Greenland forcibly (see, e.g., Washington Week). This article explores that issue.
Article 5 of the North Atlantic Treaty
The 1949 North Atlantic Treaty (also called the Washington Treaty) established the NATO Alliance and set the terms according to which the Parties to the instrument (the “Allies” in NATO parlance) would respond to an “armed attack” on an Ally in collective defense. The key provision is Article 5:
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will 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 restore and maintain the security of the North Atlantic area.
As expressly confirmed in Article 5, the authority to act collectively derives from Article 51 of the UN Charter, which had been adopted four years earlier In relevant part, Article 51 provides: “Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.” Since Article 5 is grounded in the Charter’s Article 51, action taken based on the former is subject to, at least, the same conditions and limitations as self or collective defense under the latter.
The Article 5 Geographic Limitation
The Article 5 commitment is limited to action taken against allies in “Europe or North America.” Article 6 clarifies that this includes “an armed attack on the territory of any of the Parties in Europe or North America.” As Greenland is part of North America, the sole question vis-à-vis the geographical limitation is whether military operations into Greenland would be an armed attack on Denmark.
Much has been made of Greenland’s unique status. The territory 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). It is clear that Greenland falls within the geographical coverage of Article 5.
The Article 5 Trigger
Only an “armed attack” triggers the Article 5 obligation. Lesser unlawful actions do not suffice. For instance, the U.S. actions are already in violation of the prohibition on “intervention” into the internal or external affairs of another State. As noted by the International Court of Justice in its Paramilitary Activities judgment, intervention occurs when a State coerces another with respect to its “domaine réservé,” that is, matters left by international law to States (¶ 202). Trump’s “easy way or hard way” threat is paradigmatically coercive, for, as the Court noted, the use of force is a “particularly obvious” form of coercion (¶ 205). Moreover, the Court singled out “choice of a political … system” as a paradigmatic example of a matter falling within a State’s domaine réservé; whether a State retains sovereignty over territory is perhaps the most extreme manifestation of political choice. Despite this clear violation of international law, the U.S. intervention does not trigger Article 5.
Nor does the U.S. violation of the prohibition on the threat of the use of force set forth in Article 2(4) of the UN Charter: “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). To violate the prohibition, the threat must be to engage in force that violates international law (Nuclear Weapons, ¶ 47). Such a threat must also be coercive, communicated, and credible to be unlawful (see my discussion of intervention here).
It is manifest that the U.S. threat to use force to seize Greenland is unlawful. After all, Article 2(4) specifically cites a use of force against “territorial integrity” as a basis for qualifying as a prohibited action (see also Friendly Relations Declaration). There are two bases for the lawful use of force against another State: UN Security Council authorization, which has not been granted, and self-defense under Article 51. Although the administration has claimed to need Greenland for defensive purposes, forcible defensive actions under Article 51 must respond to an imminent or ongoing armed attack. In this case, Denmark (and no other third State) has engaged in no hostile action against the United States. The threatened use of force by the United States is, as explained, coercive; multiple senior officials have communicated it; and in light of recent U.S. operations against Venezuela, the credibility of such threats cannot be dismissed as purely rhetorical.
But again, this U.S. violation of international law against Denmark is completely insufficient to activate Article 5. Nevertheless, Denmark could communicate a request for assistance from other Allies below the level of a use of force pursuant to Article 4 of the North Atlantic Treaty, which provides for consultations among the Allies whenever, in the opinion of any of them, an Ally’s territorial integrity, political independence, or security is threatened. Doing so is not a precondition for action taken pursuant to Article 5.
The condition precedent to taking measures under Article 5 is strictly limited to situations in which there has been an “armed attack.” The United States has traditionally adopted an expansive interpretation of “armed attack,” taking the position that any unlawful use of force qualifies as such (DoD Law of War Manual, §1.11.5.2; Taft, p. 300). This position diverges from that of most States in the world, including NATO members, and from the jurisprudence of the International Court of Justice, which limits armed attacks to the “most grave” forms of the use of force (Paramilitary Activities, ¶ 191).
That difference in approach is immaterial in this case. Any military operation sufficient to seize Greenland would unquestionably meet even the higher “most grave” threshold, thereby qualifying as an armed attack for Article 5 purposes. Indeed, the UN General Assembly’s Definition of Aggression Resolution, which was adopted by consensus, labels the “invasion or attack by the armed forces of a State of the territory of another State, or any military occupation, however temporary, resulting from such invasion or attack, or any annexation by the use of force of the territory of another State or part thereof,” as “aggression” (Res. 3314, art. 3(a)). Moreover, NATO’s own explanation of collective self-defense observes, “[a]n obvious example [of an armed attack] would involve an invasion by one state of the territory of another.”
A Legal Obligation to Assist?
According to Article 5, Parties to the North Atlantic Treaty “agree” to assist States that are the victims of an armed attack. This is a legal obligation, not a merely hortatory one. As a legal obligation, it must be fulfilled in good faith and in a manner consistent with the object and purpose of the North Atlantic Treaty (Vienna Convention, arts. 26 and 31(1); Gabčíkovo–Nagymaros, art. 142). Accordingly, a refusal to assist Denmark in the face of a U.S. attack would amount to an “internationally wrongful act” (Articles on State Responsibility, art. 2). In this sense, Article 5 differs from Article 51. Whereas the latter is permissive, the former is obligatory. In other words, while States could individually voluntarily agree to assist Denmark on an ad hoc basis, including by the use of force, under Article 51 of the UN Charter, they shoulder the obligation to “assist” under the North Atlantic Treaty. A key question, then, is whether the obligation to assist includes the use of force. As explained below, it does not.
Significantly, Article 5 imposes no limitations on whom collective defense may be directed against. In fact, in the aftermath of the 9/11 attacks on the United States, NATO invoked Article 5 for the only time in response to an attack by a non-State actor, al-Qaeda. There is no basis for concluding that the obligation would not extend to a U.S. attack on Denmark in the form of operations to seize control of Greenland.
As with any action in collective defense under the Charter, the State facing the armed attack, in this case, Denmark, has to request assistance (Paramilitary Activities, ¶¶ 196 and 199; Oil Platforms, ¶ 51) before other States may act in collective self-defense. For the Alliance, that request typically would be made to the North Atlantic Council (NAC). The Allies, in their role as members of the NAC, would be obligated by the principle of good faith to act on the request in a manner consistent with the treaty’s object and purpose–to lend assistance that renders the defense of an Ally against an armed attack effective. The nature and degree of assistance afforded by the other States would be limited to that requested by Denmark.
Who Authorizes Action under Article 5?
The NAC is the body that would authorize a response executed under NATO command and control. It should be noted in this regard that the North Atlantic Treaty neither expressly designates the NAC as the entity authorizing collective action nor requires decisions to be taken by consensus. It merely establishes “a Council, on which each of [the Allies] shall be represented, to consider matters concerning the implementation of this Treaty” (art. 9).
However, NATO practice has consistently treated the NAC as its core decision-making body. Moreover, the NAC has always operated on a consensus basis (see NATO’s description here and here). Importantly, because no procedural requirement of consensus appears in the treaty, a colorable argument might be fashioned that the NAC could act over U.S. objection, if only because it conducted the armed attack. That said, doing so would be institutionally unprecedented (as would an attack by one Ally against another).
It is essential to emphasize that the absence of NAC authorization would not extinguish the Article 5 obligation to provide assistance in collective defense. That obligation could be satisfied through coordinated or unilateral action outside NATO command structures. If the assistance involved the use of force, it would be subject to the self-defense requirements of necessity and proportionality (Paramilitary Activities, ¶¶ 194, 237; Nuclear Weapons, ¶ 41; Oil Platforms, ¶¶ 43, 73-74, 76).
What Assistance is Required?
While assistance is a binding obligation (subject to the request of the victim State) in the event of an armed attack, Article 5 does not require any particular form or level of assistance. This is clear from the “such action as [the assisting State] deems necessary” text in the article. In particular, it need not involve support at the level of a use of force or otherwise involve participation in the hostilities. The practice of individual Allies in the aftermath of the NAC’s determination that Article 5 applied to the 9/11 attacks is illustrative, as support ranged from overflight, intelligence sharing, and airspace surveillance to robust military action.
As I have noted elsewhere, such flexibility is not a defect in the treaty but instead a structural feature that allows the Alliance to function despite differing national threat perceptions and domestic legal and political processes for providing assistance. Thus, while a failure to provide any assistance to the victim State upon request could not be reconciled with Article 5 as a matter of treaty law, there is no quantitative or qualitative threshold of assistance beyond the requirement that the Allies act, as described above, in good faith in an effort to restore and maintain the security of the designated region.
Could the United States be Expelled from NATO?
Unlike the UN Charter, which provides for the expulsion of a member that has “persistently violated the Principles” of the UN Charter (art. 6), the North Atlantic Treaty, while allowing States to withdraw (art. 60), contains no comparable provision (on the issue of NATO expulsion (see Aurel Sari‘s excellent analysis). Therefore, the sole course of action other Allies could take in the face of the U.S. threats or subsequent military operations against Denmark would be to suspend or terminate the operation of the treaty on the basis that the United States is in “material breach.” The relevant law appears in Article 60 of the Vienna Convention on the Law of Treaties, which reflects customary international law (Gabčíkovo–Nagymaros, ¶ 46), an important point given that the United States is not a Party to the instrument.
A material breach involves “the violation of a provision essential to the accomplishment of the object or purpose of the treaty” (art. 60(3)). To the extent that the object and purpose of the treaty is the maintenance of “stability and well-being in the North Atlantic area” (preamble), and States have committed to assist each other to defend against armed attack, threatening an armed attack, let alone engaging in one, self-evidently amounts to a material breach. The decision to suspend or terminate would have to be unanimous (art. 60(2)(a)), although Denmark, as a “party specially affected,” could invoke the breach as a ground for suspension of the operation of the treaty as between itself and the United States (art. 60(2)(b). Suspension would entail the termination, in whole or in part, of rights and obligations under the treaty as between the United States and all other Allies; termination would permanently extinguish those rights and obligations. In such a case, North Atlantic Treaty obligations would continue in force for all other Allies.
Of course, suspending or terminating the obligation to assist the United States pursuant to Article 5 would be meaningless in the face of a U.S. armed attack on Denmark. However, it would probably sound the political death knell for U.S. participation in the Alliance. More immediately, it could help counter any argument that a U.S. objection in the NAC could bar NATO action.
Concluding Thoughts
That this essay addresses whether other Allies would be obligated to assist in the collective defense of Denmark should the United States launch an armed attack against Greenland is astonishing. But it is no less remarkable that the United States would even threaten to use force against a NATO Ally that has fought at its side in recent conflicts in Afghanistan, Iraq, Syria, and elsewhere. Indeed, it is worth remembering that the per capita death rate of Danish troops in Afghanistan was on par with that of the United States, and those troops were there in the collective defense of the United States.
But considering recent events, it sadly makes sense to understand what Article 5 requires of Alliance members, and what it does not. Whether the Allies would come to Denmark’s defense if the United States attacked Greenland is uncertain, as is the manner in which they might do so. Nevertheless, it is clear that, in the event of a (hopefully unlikely) U.S. armed attack, the North Atlantic Treaty obligates other Allies to assist Denmark in collective defense should Denmark so request. That obligation would have to be performed in good faith and in light of the treaty’s object and purpose, although it leaves States discretion as to the form that assistance takes. Neither the identity of the attacker nor NATO’s internal decision-making practices negate that obligation.
FEATURED IMAGE: NATO Leaders join King Willem-Alexander and Queen Máxima of the Netherlands for a family photo as they participate in the 2025 NATO summit on June 24, 2025 in The Hague, Netherlands. (Photo by Haiyun Jiang-Pool/Getty Images)
