In the wake of 11 September 2001, the Western world united in its condemnation of Osama bin Laden on the grounds that he is a terrorist operating outside the realms of International Law. But bin Laden himself has no interest in being judged according to International Law and instead claims to be guided by Islamic Law. This is the basis of his appeal to an unknown but appreciable number of Muslims throughout the world. In the West there has been little rigorous analysis of his argument in terms of its theological validity. What discussion there has been has been typified by clichŽ and stereotype either vilifying Islam or denying bin Laden any theological credibility. Neither of these approaches has helped Westerners to inform themselves of bin Laden's appeal to Muslims or shape an effective response to his actions. This article explains how Islamic Law is formulated and studies the development of Islam's approach to warfare -- jihad doctrine. Bin Laden's fatwa is then compared against that doctrine to analyse if it is consistent with Islamic Law. The analysis shows that there is justification within Islamic Law to call for a defensive war against perceived external aggression. But there is absolutely no justification in Islamic Law for the means advocated by bin Laden in his fatwa -- the killing of non-combatants wherever they may be found. These findings are relevant to the West's response to bin Laden.