Regulating Military Force Series – The Meaning of Prohibited “Use of Force” in Article 2(4) of the UN Charter

Editors’ note: The author delivered remarks on the subject of this post at the conference “International Law and the Regulation of Resort to Force: Exhaustion, Destruction, Rebirth?” at the Centre for International Humanitarian and Operational Law, Palacký University in Olomouc, Czech Republic.

Although the prohibition of the use of force in Article 2(4) of the UN Charter and customary international law is a cornerstone of the modern international legal system and considered by many to be a peremptory norm, the definition of “use of force” is legally uncertain. Grey areas include: cyber operations; purported exercise of maritime law enforcement that is excessive (e.g. Red Crusader, M/V Saiga No. 2) or without a lawful basis (e.g. Fisheries Jurisdiction (Germany v. Iceland); Guyana v. Suriname; Rainbow Warrior); attempted targeted killings (e.g., Sergei Skripal in the UK in 2018); and counterspace operations such as targeting satellites through kinetic attack, jamming, dazzling, or rendezvous and proximity operations.

When faced with such acts, how can a legal adviser (or legal scholar) evaluate whether an act constitutes a prohibited use of force under Article 2(4) of the UN Charter and customary international law? Open questions surround the definition of prohibited use of force, including: does “force” mean physical/armed force only? What type of physical effect is required? Do temporary or potential effects count? Is there a gravity threshold, and if so, how low is it? Is a coercive or hostile intent required? In addition, there is uncertainty over the interpretation of the contextual elements that bring a use of force within the scope of the prohibition, such as when a use of force is considered to take place in “international relations.” So far, there is no common understanding of the interpretation of a prohibited use of force or how to apply it in a given situation.

Why It Matters

This lack of clear interpretation is deeply problematic for a norm that determines the lower threshold and applicability of the international legal prohibition of the use of force between States in their international relations (jus contra bellum). Crucially, the definition of “use of force” also determines the size of the gap between a prohibited use of force and an “armed attack,” giving rise to a right of self-defence under Article 51 of the UN Charter and customary international law. There is controversy over the threshold of an armed attack, but less attention has been paid to the threshold of a use of force. How high or low this threshold is determines the size of the gap between Articles 2(4) and 51, which has important security and policy consequences.

Acts reaching the threshold of prohibited force cannot be used as lawful countermeasures and can only be justified by self-defence under Article 51 of the UN Charter and customary international law, or on the basis of UN Security Council authorisation under Chapter VII. To the extent that the prohibition of the use of force is a peremptory norm (jus cogens), acts that fall within its scope cannot be legally excused by circumstances precluding wrongfulness, such as necessity, distress, and force majeure. Acts which violate the prohibition give rise to State responsibility under international law. For serious violations of the prohibition, special consequences arise. It is therefore important to be able to assess whether a particular act falls within the scope of the prohibition of the use of force or is governed by other international legal frameworks, such as law enforcement.

Elements and Definition of “Prohibited Force”

In my recently published book, presented at the Resort to Force conference in Olomouc, Czech Republic, in September 2023, I seek to address this problem of uncertainty over the interpretation of prohibited use of force by identifying its elements and proposing a definitional framework that can be applied in practice. In this post, I give a brief overview of the definitional framework for prohibited force that I develop in my book and apply it to a concrete example in the domain of outer space to illustrate how it can be applied in practice.

Prohibited force under Article 2(4) of the UN Charter comprises both contextual elements (“All Members, in their international relations, against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations”) and elements of “use of force” as such. All contextual elements must be present for a use of force to fall within the scope of the prohibition.

The elements of “use of force” identified in my book are: (1) means, i.e., physical force; (2) physical effects, including direct physical effects that may be either permanent or temporary, as well as actual or potential in nature; (3) an object or target with a sufficient nexus to another State; (4) the gravity of effects; and (5) intent, or put differently, that an intended action is required, as opposed to an accident. There is textual support for the position that a coercive intent is required under article 2(4).

I address the interpretation of the contextual elements of prohibited force (with a focus on “international relations”) and the elements of a use of force in detail in the second part of my book. In this post, I focus on the definitional framework bringing together these elements.

Unlike other legal concepts (notably, domestic and international crimes), I propose that a “use of force” under Article 2(4) of the UN Charter describes a type, meaning that rather than consisting of a checklist of necessary and sufficient elements (as for elements of crime), it consists of a basket of elements which must be weighed and balanced to determine whether the threshold of the definition is met. This means that not all elements must be present for an act to constitute prohibited force if they are compensated by other elements. For example, a hostile or coercive intent may turn a forcible act into a use of force even if other elements are relatively weak, such as a low gravity or if the harm is only potential but unrealised. I illustrate this idea briefly by applying the definitional framework to a concrete scenario regarding use of force in the emerging military domain of outer space.

Case Study: Kinetic Anti-Satellite Weapons Test

Outer space is an increasingly important military domain, where emerging technologies are already being tested and used in counterspace operations. Given the substantial (and so far, relatively under-recognised) military, commercial and civilian dependence on outer space, it is necessary to understand how to apply existing legal frameworks including the jus contra bellum in this domain. A key counterspace capability that has already been tested by China (2007), the United States (2008), India (2019) and Russia (2021) is direct-ascent anti-satellite (ASAT) weapons. These tests create a large amount of dangerous space debris that can damage and destroy other States’ satellites (military and commercial). If such a test generates space debris that destroys another State’s satellite, could this violate the prohibition of the use of force?

Applying the definitional framework set out above, the use of force elements that are present are physical means, physical effects, and a high gravity. The element missing in this hypothetical scenario is a hostile or deliberate intent. However, given the known dangers of space debris, deliberately creating debris which causes damage to another State’s space object is unlikely to be seen as a mere accident or mistake, and at the very least will be regarded as reckless. Does this combination of elements suffice to reach the threshold of a prohibited use of force? Perhaps not. But if some of the elements are more heavily weighted in the scenario, such as secondary effects with a high gravity intensity (for example, if a satellite that carries out key military or civilian functions is destroyed by the debris) and the conduct evinces a particularly reckless or hostile intent (for example, the ASAT test is unannounced and conducted at high altitude and there are pre-existing tensions between the State conducting the ASAT test and the State whose satellite is destroyed by the debris), then all of these elements in combination could meet the threshold.

Thus, in certain circumstances, the creation of debris by a State conducting a direct-ascent ASAT (DA-ASAT) test which then damages or destroys the space object of another State could constitute a prohibited use of force in violation of Article 2(4) of the UN Charter and customary international law. Whether such damage occurred in international relations will be influenced by the perceived intent of the State conducting the DA-ASAT test and the degree of recklessness in carrying out the strike.

Conclusion 

For a rule that is the “cornerstone” of the international legal order and a peremptory norm of international law, the lack of legal certainty over the definition of prohibited use of force between States is unsatisfactory and dangerous. This ambiguity can be exploited by States through grey-zone operations designed to test the limits of sub-threshold acts and gives rise to security risks, including the risk of escalation or miscalculation. The definitional framework for prohibited force that I propose in my book is an attempt to overcome this problem and propose a solution that can be applied in practice. A clear definition of prohibited use of force is especially useful to assess and classify acts of lower gravity, and those that use emerging technologies or take place in newer military domains such as outer space. Although there is room for debate regarding the elements of a prohibited use of force and their combined threshold, this framework offers a shared language which can bring more clarity to the debate and provide guidance when applying Article 2(4) of the UN Charter in practice.

***

Dr Erin Pobjie is Assistant Professor of International Law at Essex Law School, UK and a Senior Research Affiliate at the Max Planck Institute for Comparative Public Law and International Law in Heidelberg, Germany. The views expressed in the publication are the sole responsibility of the individual author. They do not necessarily reflect the views or opinions of the United Nations, UNIDIR, its staff members or sponsors.

Photo credit: Pexels, CUP