An obscure Arabic word is making a comeback from centuries of oblivion to dominate the debate about whom Muslims are allowed to kill in the service of political goals. The debate has been triggered by the killing of large numbers of Muslims, including women and children, by Islamist insurgents in Iraq. Are such acts permissible? Judging by fatwas (religious opinions) and articles by Muslim theologians and commentators, the Islamic ummah (community) is divided on the issue. Those who believe that killing innocent people, including Muslims, is justified in certain cases, base their opinion on the principle of tattarrus. The word,...