The acts of lawlessness committed on September 11, 2001 were followed by a #war on terror#. This book sets out the essential features of the international legal framework against which the #9/11# attacks and the lawfulness of measures taken in response thereto fall to be assessed. It addresses, in an accessible manner, relevant law in relation to: #terrorism#, questions as to #responsibility# for it, the criminal law framework, lawful constraints on the use of force, the humanitarian law that governs in armed conflict, and international human rights law. It indicates the existence of a legal framework capable of addressing events such as #9/11# and governing responses thereto. The author examines the compatibility of the #war on terror# with this legal framework, and questions the implications for states responsible for violations, for third states and for the international rule of law.