Though peace is the ideal of all the great religions, warlike behavior by their practitioners is the reality. But has there ever been a war of religion, a head-to-head encounter in which the goal is to decisively harm or obliterate the other? Or has religion mainly been used as a pretext or rationale for interstate or inter-ethnic wars? Can one distinguish a war of ethnicity from a war of religion? Is genocide a war of religion or ethnicity, or is it collateral damage to a war between states? Are terrorist groups such as al-Qaeda, ISIS, and Boko Haram conducting wars of religion? Karen Armstrong's Fields of Blood will provide the basis for answering these and other questions.