Abstract: Any analysis of international law and the ‘war on terrorism' following the events of 11 September 2001 needs to start with recognition of the fact that the terrorist atrocities perpetrated in the United States on that date were illegal. On that, international lawyers the world over agreed. Whatever lay behind those terrible events, there was no legal justification for them and none has been offered. Yet the consensus about the illegality of the terrorist attacks did not lead a similar consensus about legal questions raised by the US reaction to them. The legality of the United States' resort to force against al Qaeda and the Taliban regime in Afghanistan, of the conduct of the hostilities which followed, and of the status and treatment of prisoners held by the United States at the naval base at Guantanamo Bay have all been matters of controversy. This chapter is divided into three parts. The first examines how international law characterizes the events of 11 Septembershould they be seen simply as crimes, or are they a threat to international peace or an armed attack or act of war? The second part considers the legality of the resort to force by the United States of America and the United Kingdom in Afghanistan. It examines the legal justification offered by the two governments and discusses whether that justification was sufficient. The third part reviews the law applicable to the conduct of the hostilities in Afghanistan, including the legal requirements regarding targeting and the treatment of prisoners.