Since men first banded together in opposing groups for the purposes of war, combatants on both sides have entered every fray convinced that only their own cause is favored by whatever god or gods they honor.

During the First Crusade, for example, the knights of Christ rallied to cries of " Dieu li volt! " ("God wills it!"), heralding their belief as to whose side God was on. At the same time, the warriors of Islam, confident of the sanctity of their own cause, rushed into battle shouting "Death to the infidels!" It is, after all, written in the Koran that "the true believers fight for the cause of Allah, but the infidels fight for idols" (4:73).

Both Christians and Muslims placed their faith in one god and fought in the name of that god; both sides were unswervingly convinced of the rightness of their...