The Prohibitions of Threat of Force Revisited: Greenland, Credibility and International Law

In the beginning of January, following reported United States actions against Venezuela and threats by President Trump against Cuba, Colombia and Mexico, there were renewed statements by President Trump regarding the annexation of Greenland. President Trump stated that “we need Greenland from the standpoint of national security”. On 6 January, it was reported that the White House stated the President and his advisers were discussing a range of options to acquire Greenland and that the use of force “is always an option at the commander-in-chief’s disposal.” Although a similar narrative dominated the news last year, the present context has placed threats of taking over Greenland in a new perspective. On Monday, 5 January, the Danish Prime Minister publicly urged President Trump to “stop the threats” concerning Greenland, stressing that “the United States has no right to annex any of the three nations in the Danish realm”. After these developments, further statements by President Trump and senior advisers reinforced the impression that military options were being seriously contemplated.

These statements by US officials regarding Greenland – and potentially also statements directed at Cuba, Mexico and Colombia – implicate a foundational norm of international law: the prohibition not of the use but of the threat of force. This post will argue that the threats articulated by the US government regarding the possible military acquisition of Greenland violate this prohibition. Specifically, it compares last year’s threats to those posed in 2026, arguing that the change of context and specific threats have now met this threshold without any doubt. 

It must be noted that following diplomatic engagements at the World Economic Forum in Davos, subsequent statements suggest that the possible use of force against Greenland is no longer being pursued. In his address at the Forum on 21 January, President Trump said “I won’t use force”. This apparent de-escalation, however, does not negate the legal relevance of the earlier statements. Under international law, threats of force must be assessed in light of the circumstances prevailing at the time they were made, rather than on the basis of later political developments. 

Threat of force

Article 2(4) of the United Nations Charter does not only contain the prohibition on the use of force but also related to threats of force. The prohibition of the threat of force, just like the use of force, is considered by both the International Court of Justice (ICJ) and the United States itself as a rule of customary international law and as such, is binding upon all States. Although explicit invocations of the prohibition of threats of force are relatively rare in State practice, perhaps because credible threats often precede actual uses of force – the most authoritative treatment of the norm is found in the ICJ’s 1996 Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons (Nuclear Weapons Advisory Opinion). In this Advisory Opinion, the Court was asked to answer the question: “is the threat or use of nuclear weapons in any circumstances permitted under international law?”. In addressing this question under Article 2(4) of the Charter, the Court examined nuclear deterrence and the signaling of nuclear capabilities. In Paragraph 47, the Court Stated:

Whether a signaled intention to use force if certain events occur is or is not a ‘threat’ within Article 2(4) of the Charter depends upon various factors. If the envisaged use of force is itself unlawful, the stated readiness to use it would be a threat prohibited under Article 2(4).

This is further elaborated upon in paragraph 48:

Some States put forward the argument that possession of nuclear weapons is itself an unlawful threat to use force […]. Whether this is a ‘threat’ contrary to Article 2(4) depends upon whether the particular use of force envisaged would be directed against the territorial integrity or political independence of a State, or against the Purposes of the United Nations, or whether, in the event that it were intended as a means of defense, it would necessarily violate the principles of necessity and proportionality.

Accordingly, the legality of a threat hinges on the legality of the use of force it implies. In addition, legal scholarship has identified further criteria relevant to determining whether a statement or course of conduct qualifies as a prohibited threat under Article 2(4) (summarized here). Chief among these is credibility (see e.g. Starchier, p. 259 and Gill & Tibori Szabo, p. 60). 

Inherently, ascertaining credibility is heavily dependent on perception. According to the ICJ, a threat amounts to a “signaled intention” to use force or a stated readiness to use it (Nuclear Weapons Advisory Opinion para. 47). This stated readiness must have some degree of credibility, according to the International Law Commission (at para. 57), and there needs to be “good reason to believe” that a use of force is “seriously contemplated”. This is the only way that a threat can be distinguished from “mere verbal excesses”. 

Scholarship has developed several conditions to assess this credibility, often framed in terms of specificity, proximity, and directness. Specificity refers to the clarity with which the threatened use of force is described. Proximity concerns temporal closeness: that is whether the threatened action appears imminent (see Stürchler, pp. 37-54). Directness considers whether the statement clearly links the use of force to a particular demand or condition, or whether it remains rhetorical or ambiguous. There is disagreement as to how demanding these thresholds should be. Some scholars insist that only specific and imminent statements should count as threats, while others see even less determinate language, especially if backed by military capabilities, as sufficient to fall within the prohibition (See e.g.  Hofmeister). Others argue that the development of military capabilities without an attached threatening communication, regardless of context, cannot be considered as a threat to use force (for example: Rule 12 Tallinn Manual).

In practice, credibility focuses on the broader political context, capability of the threatening State and audience interpretation. This point is underlined by Grimal and Kleczkowska (Grimal, pp. 44-45; Kleczkoswka, pp. 19-20). What for one State is a clear threat may be perceived by another State as merely an unfriendly act, and thus a case-by-case assessment is required, taking into account the elements referred to above. One relevant factor in this context is a State’s past record of conduct: has it followed up threats to use force with military action in the past? (See Jankovic & Roeben, p. 826).  

Threats now versus threats then

This raises the question why the present statements concerning Greenland may amount to a violation of international law, whereas similar remarks made in May 2025 did not attract the same legal characterization, at least by Denmark itself. Previously, President Trump’s comments regarding Greenland were widely dismissed as politically motivated rhetoric rather than expressions of an actual strategy. Specifically, the EU Commission’s spokesperson stated that “we are indeed speaking about something extremely theoretical on which we will not want to elaborate”, when asked about Trump’s threats in early 2025. Furthermore, the US Congress openly stated it would not back a Greenland invasion. In this case, the Danish Prime Minister clearly stated herself that “I don’t have the fantasy to imagine that it’ll ever get to that”, when asked about the feasibility of the US annexing Greenland. As such, even the ‘victim Head of State’ brushed the threat off. The same perception applied to broader US patterns of threats directed at other States, which were not followed by military action (See Corten, p. 124). Trump has had a history of threatening a number of nations, without following up with military action, and as such, the “credibility of the threat” would be perceived as much lower. In terms of the criteria discussed below, earlier statements scored low on all three elements: they lacked specificity, proximity and directness. They were more easily classified as “verbal excesses” rather than a “signaled intention to use force”. Audience interpretation confirmed this: both Denmark and the EU expressly characterized the remarks as theoretical. 

These conditions have shifted over the past few days. First, Denmark’s Prime Minister has now explicitly called upon the US to stop its threats, affirming that the US has no legal right to take over Greenland. In response, the White House publicly announced it is discussing options to acquire Greenland including using military options. Furthermore, multiple European leaders have stated that they are working on a joint response should the US take action against Greenland. Secondly, recent US actions against Venezuela have materially altered the broader context in which President Trump’s statements are to be assessed. This significantly enhances the credibility of subsequent threats, including those directed at Greenland. Indeed, the response by many world leaders shows that this threat is considered as credible and further underscores the illegality of this behavior.

In this situation, the conditions for credibility have more clearly been fulfilled. Statements are more specific, referring to possible military operations and proximity has increased because action is under active consideration by the US administration. Combined with a changed record of state conduct, these factors shift the perception of credibility. Indeed, the response by many world leaders shows that this threat is considered as credible and further underscores the illegality of this behavior.

Nevertheless, as the ICJ made clear, credibility and intent alone are not decisive. The legality of the situation ultimately depends on whether the threatened conduct – here, annexation – is itself lawful under international law. 

Legality of an annexation of Greenland

The United States is currently openly discussing the annexation of Greenland. Under international law, annexation means the forcible acquisition of territory by one State at the expense of another State. Annexation is no longer recognized as a legally admissible mode of acquisition of territory, as it violates the prohibition of the threat or use of force ( ICJ Wall Advisory Opinion, para. 87). It follows that the annexation of Greenland would constitute an unlawful use of force against Denmark. Consequently, a credible threat to annex Greenland would itself amount to a prohibited threat of force under Article 2(4). While exceptions and alternative legal pathways are sometimes discussed by scholars – such as cession or the exercise of self-determination – these all hinge on valid consent. In the present case, there is no indication of consent by the Greenlandic population, nor by Denmark. Absent such consent, annexation would be unlawful. 

Conclusion

The developments of the past weeks mark a significant shift in the legal assessment of United States foreign and security policy rhetoric. Whereas earlier threats directed at sovereign States were often dismissed as politically motivated posturing, recent actions have demonstrated a willingness to follow up threats with force. This has substantially increased the credibility of President Trump’s statements concerning Greenland.

Although recent developments surrounding the World Economic Forum suggest that the immediate use of force against Greenland is currently unlikely, this does not retroactively affect the legal assessment of earlier threats. The fact that force may ultimately not be used does not render prior credible threats legally irrelevant or retroactively lawful. 

International law prohibits not only the use of force, but also threats to use force where the threatened action would itself be unlawful. The annexation of Greenland would violate core principles of the international legal order, including the prohibition of the use of force and respect for territorial integrity. Accordingly, a credible threat to annex Greenland is likewise prohibited. Compared to last year, the changed context, combined with explicit objections from Denmark and heightened concern within the international community, renders the present threat both more credible and more clearly in violation of international law.