Shades of Illegality in the Law Against War

Abstract This book addresses breaches of international law and disputes over norms governing the prohibition of the use of force. The aim is to facilitate an understanding of the dynamics and resilience of international law, but also the dangers of its possible disintegration. It explores what is here described as the shades of illegality. It unpacks the phenomena related to illegal actions by developing a typology of illegality that allows us to distinguish the different forms of illegality and their specific effects on international legal norms and the international legal order in general. Six types of illegality are developed throughout the book, spanning from illegality occurring in cases in which actors agree on the applicable law, but dispute its application to the case at hand; to cases in which States break the law in emergency situations; to cases in which the law itself remains unclear as to its general scope and content; to cases in which States break the law in order to change particular rules of international law; and finally, to situations that are described as systemic opposition, that is, States actively aiming to overcome or change foundational rules of the international system. The book shows that not all unlawful acts are problematic as such; rather, they may even have a constructive role in the overall functioning of the international order. However, it draws attention to cases in which breaches of law articulate a systemic opposition to existing law and highlights current centrifugal forces that pose a threat to the law against war.