Necessity and Proportionality in International Peace and Security Law
The Lieber Studies Series

Necessity and proportionality hold a firm place in the international law governing the use of force by states, as well as in the law of armed conflict. However, the precise contours of these two requirements are uncertain and controversial. The aim of Necessity and Proportionality in International Peace and Security Law is to explore how necessity and proportionality manifest themselves in the modern world under the law governing the use of force and the law of armed conflict, and how they relate to each other. The book explores the ways in which necessity and proportionality are applied in practice and addresses pressing legal issues in the law on the use of force, including the controversial "unwilling and unable" test for the use of force in self-defense, drones and targeted killing, the application of this legal regime during civil war, and the need for further transparency in states' justification for the use of force in self-defense. The analysis of the role of military necessity within the law of armed conflict on the modern battlefield focuses on the history and nature of the principle of military necessity, the proper application of the principle of proportionality, how commanders should account for mental harm in calculating proportionality, and the role artificial intelligence and autonomous weapons systems may play in proportionality analysis. The book concludes with a discussion of the potential role of proportionality in the law governing post-conflict contexts.

Claus Kreß is a Professor of International Law and Criminal Law. He holds the Chair for German and International Criminal Law and is the Director of the Institute of International Peace and Security Law at the University of Cologne. His prior practice was in the German Federal Ministry of Justice on matters of criminal law and international law. In addition to his scholarly work, comprising more than 150 publications on the law on the use of force, the law of armed conflicts and international criminal law, he has been a member of Germany's delegations in the negotiations regarding the International Criminal Court since 1998. He is a Life Member of Clare Hall College at the University of Cambridge, a Member of the Academy of Sciences and Arts of Northrhine-Westfalia, and the recipient of the 2014 M.C. Bassiouni Justice Award. He holds honorary doctorates from the State University Tbilisi and the University of Huánuco. Robert Lawless is a Judge Advocate in the United States Army Judge Advocate General's Corps. He has been assigned as both a prosecutor and defense counsel, litigating military justice cases in courts-martial. He also served as an administrative and operational law attorney for the Third Infantry Division during Operation Resolute Support in Afghanistan. His assignments also include serving as an Assistant Professor in the Department of Law at the United States Military Academy, West Point, New York.