Accountability challenges for the crime of aggression: assessing legal and political mechanisms in response to Russia's invasion of Ukraine

Abstract

The Russian invasion of Ukraine, along with the debate regarding the crime of aggression has exposed the structural limitation of international law in ensuring accountability for grave violations. Despite the prohibition on the use of force enshrined in the United Nations (UN) Charter, the enforcement methods that can be used against states committing acts of aggression are inadequate. This thesis aims to assess the extent to which existing legal and political mechanisms are capable of ensuring accountability for the crime of aggression in the context of Ukraine and to examine whether the responses of the European Union (EU) and North Atlantic Treaty Organization (NATO) can operate as complementary forms of accountability. The analysis focuses on the development of the crime of aggression in international law, the jurisdictional constraints of the International Criminal Court (ICC), the legal and political debate surrounding the establishment of a Special tribunal and the role of EU and NATO responses, alongside the broader political constraints affecting enforcement. The findings suggest that the ICC’s capacity to address the crime of aggression remains significantly constrained, particularly due to the jurisdictional and political limitations. Accountability processes are therefore heavily shaped by geopolitical considerations and power asymmetries. In this context, the paralysis of the United Nations Security Council (UNSC) created space for regional organisations such as the EU and NATO to assume a leading role in responding to aggression. Through sanctions, political coordination, military assistance and diplomatic pressure, these organisations contribute to accountability indirectly, functioning as complementary rather than judicial accountability mechanisms. The proposal for a Special Tribunal reflects an attempt to address existing enforcement gaps but also illustrates the tension between legal design and political feasibility. The thesis concludes that the main challenge of the international criminal law lies not in the absence of legal norms, but in the persistent gap between normative aspirations and political reality, which continues to shape the effectiveness and equality of accountability mechanisms.