In the war on terror, both 'sides' have taken great pains to justify their actions in moral terms. As force is employed so are sophisticated arguments which directly invoke the just war traditions of the West and Islam. This book offers an exploration of the ways in which George W. Bush and Osama bin Laden drew upon, and simultaneously re-conceptualised, important moral concepts from within the Western and Islamic just war traditions. It examines a range of jus ad bellum and jus in bello issues, including Western debates about pre-emptive self-defence, Islamic requirements for right authority to declare war, proportionality in the Battle of Fallujah, and the legitimacy of suicide bombing in Islam. It also considers how a series of authoritative voices in the West and in the Muslim world appealed to just war and jihad ideas to vigorously contest Bush and bin Laden's cases for war. The book's central argument-that the Bush administration and al-Qaeda departed from important consensuses about justified warfare-contains within it an alternative way of understanding the war on terror. Rather than a clash between civilisations, it is suggested that the conflict can be accounted for by a clash within civilisations: in resorting to war, both sides acted against their own traditions and contravened the requirements of their own civilisations. This book is a project of the Oxford Leverhulme Programme on the Changing Character of War.