This book examines to what extent the right of self-defence, as laid down in Article 51 of the Charter of the United Nations, permits States to launch military operations against other States. In particular, it focuses on the occurrence of an 'armed attack' - the crucial trigger for the activation of this right. In light of the developments since 9/11, the author analyses relevant physical and verbal customary practice, ranging from the 1974 Definition of Aggression to recent incidents such as the 2001 US intervention in Afghanistan and the 2006 Israeli intervention in Lebanon. The notion of 'armed attack' is examined from a threefold perspective. What acts can be regarded as an 'armed attack'? When can an 'armed attack' be considered to take place? And from whom must an 'armed attack' emanate? By way of conclusion, the different findings are brought together in a draft 'Definition of Armed Attack'.

Winner of the 2011 ASIL Francis Lieber Prize

'… a great strength of the book is that it provides a detailed restatement of the law on self-defence and the crucial issues surrounding it … Ruys’ work demonstrates a large amount of research into the content of the customary law, using both an ‘incident analysis’ approach and a consideration of practice in abstracto (for example, in relation to debates over various UN declarations). This research into state practice is of notable breadth and depth, and forms the cornerstone on which all the book’s analysis is built … this is an excellent book on armed attack …'

James A. Green Source: Journal of Conflict and Security Law

'… a particularly subtle and reasoned interpretation of jus contra bellum established by the UN Charter. One could even say that this is the most rigorous and successful book to date about self-defence.'

Professor Olivier Corten Source: translated from Revue Belge de Droit International

'… 'Armed Attack’ and Article 51 of the UN Charter is a well written and carefully researched contribution which will become a bookshelf staple for academics working in the area.'

Heather A. Harrison Dinniss Source: The Modern Law Review

'Tom Ruys has written one of the most important books on the Charter law on the use of force. His study is meticulously researched, methodologically sensitive, extremely thoughtful, and elegantly written and on top of all this, it shows balanced judgment. It must therefore be included in the rather short list of significant monographs on the subject-matter … by having written this splendid book, Ruys has rendered the determinacy of this crucially important body of law a most valuable service.'

Claus Kreß Source: British Yearbook of International Law