By Lara Kajs
Dispatches from the Field—The Genocide Report
Washington, DC—6 August 2024
The crime of aggression occupies a central place in the architecture of mass atrocity crimes. Historically, large-scale atrocities rarely emerge in isolation; they are often preceded by unlawful uses of force that create the conditions for war crimes, crimes against humanity, and genocide. Understanding aggression is therefore essential to both accountability and prevention.
The Foundations of Accountability
The adoption of the Rome Statute in 1998 marked a turning point in international criminal law, establishing the International Criminal Court as a mechanism to hold individuals accountable for the most serious crimes of concern to the international community. Alongside genocide, crimes against humanity, and war crimes, the crime of aggression was recognized as a core offense—though its definition and jurisdiction were only formalized later.
Not since the post-World War II tribunals in Nuremberg and Tokyo has the international legal system sought to impose individual criminal responsibility on leaders for the decision to wage aggressive war. The inclusion of aggression within the ICC’s jurisdiction reflects a renewed effort to address the root cause of many conflicts.
Aggressive war is not only an international crime; it is the supreme international crime.” —Nuremberg Tribunal Judgment
The ICC’s Fourth Core Crime
Under Article 8 bis of the Rome Statute, the crime of aggression applies exclusively to individuals in positions of political or military authority. It is limited to those who are effectively able to exercise control over or direct state action. Criminal responsibility attaches to those involved in the planning, preparation, initiation, or execution of an act of aggression.
For conduct to qualify, it must constitute a manifest violation of the United Nations Charter, assessed by its character, gravity, and scale. This threshold ensures that only the most serious breaches—those that fundamentally undermine international peace and security—fall within the Court’s jurisdiction. Lawful uses of force, including self-defense and actions authorized by the UN Security Council, are excluded.
Defining Aggression
International law defines aggression as the use of armed force by one State against the sovereignty, territorial integrity, or political independence of another State. This definition, reflected in United Nations General Assembly Resolution 3314, encompasses a range of actions regardless of whether a formal declaration of war has been made.
These acts include invasion, military occupation, annexation by force, bombardment, and blockades. They also extend to attacks on a State’s armed forces, the use of proxy forces or mercenaries, and the facilitation of aggression by allowing one’s territory to be used for attacks against another State.
Contemporary conflicts illustrate how these principles are applied. Russia’s 2014 annexation of Crimea and its 2022 full-scale invasion of Ukraine represent clear examples of the use of armed force against the territorial integrity and political independence of another State. These actions have been widely characterized as violations of the UN Charter and raise substantial questions of individual criminal responsibility at the leadership level.
Other situations, including prolonged military occupation and settlement expansion, continue to generate legal debate regarding whether and how elements of aggression may be implicated. These cases underscore the complexity of applying the legal framework to evolving geopolitical realities.
Evaluating the Scale of Armed Force
International law distinguishes between varying armed use of force based on intensity and consequence. At the lowest level, a use of force may trigger state responsibility for violating international norms. At a higher threshold, an armed attack permits the victim State to exercise the inherent right of self-defense under the UN Charter.
The crime of aggression represents the most serious category. It requires not only the use of force, but a use of force that rises to a manifest violation of the Charter. This distinction is critical: not every unlawful use of force constitutes a crime of aggression, and not every act of aggression meets the threshold for individual criminal liability.
The International Court of Justice has emphasized that scale and effect are central to this determination, distinguishing isolated or minor incidents from actions that fundamentally threaten international peace.
Prosecuting Crimes of Aggression
Determining whether aggression has occurred requires a comprehensive assessment of the facts, including the scale, context, and consequences of the use of force. Article 2(4) of the UN Charter establishes the foundational prohibition: States must refrain from the threat or use of force against the territorial integrity or political independence of other States.
When this prohibition is breached at a sufficient scale, individual criminal responsibility may follow. The crime of aggression is unique in that it targets the leadership decisions that initiate unlawful war, rather than solely the conduct that occurs during conflict. Subsequent acts of armed force can extend the scope of the original violation and deepen individual liability.
In the case of Russia’s actions in Ukraine, the available evidence has led many legal experts and policymakers to conclude that the threshold for aggression has been met. These findings have intensified calls for accountability mechanisms capable of prosecuting those responsible at the highest levels of decision-making.
Bottom Line
The crime of aggression remains one of the most consequential yet challenging areas of international criminal law. By focusing on the unlawful use of force at its inception, it addresses the conditions that give rise to broader patterns of violence and mass atrocity.
Strengthening mechanisms to investigate and prosecute aggression is essential to closing accountability gaps and deterring future violations. Without enforcement at the leadership level, the prohibition on aggressive war risks becoming aspirational rather than operational.
Atrocity Prevention Lens
The crime of aggression functions as an early warning indicator for mass atrocities. Unlawful uses of force often create the enabling environment for large-scale violence against civilian populations. Monitoring patterns of escalation, territorial incursions, and leadership intent is critical for prevention. Effective responses require not only diplomatic engagement but also credible legal consequences for those who initiate aggressive war.
Legal Framework
United Nations Charter
Article 2(4) establishes the prohibition on the use of force, forming the legal foundation for identifying acts of aggression.
Rome Statute of the International Criminal Court
Article 8 bis defines the crime of aggression and sets the threshold for individual criminal responsibility, limiting liability to those in positions of leadership.
Customary International Law
The prohibition on aggressive war is widely recognized as a peremptory norm, binding on all States regardless of treaty participation.
Accountability Mechanisms
The International Criminal Court and potential ad hoc tribunals serve as mechanisms for prosecuting aggression, though jurisdictional and political challenges continue to limit enforcement.
Suggested Citation
Kajs, Lara. “The Crime of Aggression.” Dispatches from the Field. The Genocide Report, Washington, DC, 6 August 2024.
Photo credit: “Anti-terrorist operation in eastern Ukraine (War Ukraine) by the Ministry of Defense of Ukraine. Licensed under CC BY-SA 2.0.
About TGR
The Genocide Report (TGR) publishes analysis and educational resources on conflict, international law, and atrocity prevention. Its work seeks to bridge academic research, field realities, and public understanding of mass violence and civilian protection.
About the Author
Lara Kajs is the founder and executive director of The Genocide Report, a Washington, DC-based educational nonprofit focused on atrocity prevention and international law. She is the author of several field-based books on conflict, displacement, humanitarian crises, and international humanitarian law, drawing on extensive research and field experience in Yemen, Syria, and Afghanistan. Her writing and public speaking focus on atrocity crimes, forced displacement, the protection of civilians, and the legal frameworks governing armed conflict.