From its origins Islam has been an expansionist religion, understanding itself as a matter of faith to be in a permanent state of war with the non-Muslim world. After the initial consolidation of the Islamic caliphate, however, it soon became apparent that constant military hostilities could not be sustained and that other forms of relationship with non-Muslim nations would be necessary. To reconcile the imperatives of faith with the limits of military power, Islamic scholars developed elaborate legal doctrines. In the second century of the Muslim era (eighth century C.E.), hundreds of...