The principle of equality of belligerents mandates that the rules of international humanitarian law (IHL) apply equally to each party in an armed conflict, regardless of the legality of their use of force under jus ad bellum. This principle has been extensively analyzed in academic literature; its importance is universally recognized and its legal foundations and effects are well defined. However, this is primarily true with respect to its application in international armed conflicts (IACs) – in contrast, the principle does not receive equivalent recognition in situations of non-international armed conflict (NIAC), where at least one party is a non-State armed group. The issue arises from the lack of an accepted definition of the principle in NIACs, given the absence of any applicable international jus ad bellum in such conflicts. The present paper will try to remedy this issue by proposing that the principle is composed of two elements: symmetry of application and symmetry of substance. It will introduce this definition as it applies to IACs and NIACs and argue that the principle in NIACs is primarily defined by its symmetry of substance component. It will also evaluate the principle’s nature as a general principle of IHL and explore some of its concrete effects on IHL rules.

Common Article 3 to the four Geneva Conventions encourages the parties to a non-international armed conflict to bring into force international humanitarian law provisions through the conclusion of special agreements. Since armed groups are ever more frequent participants in contemporary armed conflicts, the relevance of those agreements as means to enhance compliance with IHL has grown as well. The decision-making process of special agreements recognizes that all the parties to the conflict participate in the clarification and expansion of the applicable rights and obligations in a way that is consistent with the principle of equality of belligerents. This provides incentives for armed groups to respect the IHL rules they have themselves negotiated. However, even upon the conclusion of such agreements, it remains unclear which legal regime governs them. This paper will argue that special agreements are governed by international law instead of domestic law or a sui generis legal regime.

The equal application of international humanitarian law (jus in bello) to all parties to an international armed conflict is a cornerstone principle of jus in bello. In his article, Professor Mandel casts doubt on the legal basis of this principle. Reacting to this claim, this contribution demonstrates that the ‘equality of belligerents’ is a principle firmly grounded in both conventional and customary international law. Moreover, its legal force withstands the test of international jurisprudence, including the International Court of Justice's controversial Nuclear Weapons advisory opinion.