To speak of “holy war” today is, at least in the West, to speak of Islam or Islamic fundamentalism. Yet, even a cursory glance at history reveals that at one time or another all of the world’s major religions have sacralized violence, at least under certain conditions. This article seeks to understand why, despite the great variety in doctrine and praxis, the world’s major religions have, and continue to, support warfare under such guises as “just war,” “jihad,” or “defense of the [Buddha] Dharma.” While recognizing the important role that doctrine, especially competing claims of exclusive truth, plays in religiously endorsed warfare, this article seeks to take a holistic look at the multiple causes of this phenomenon, including anthropological, sociological, economic, psychological and even evolutionary factors. A wide variety of historical examples from multiple faiths compliment the theoretical constructs, revealing that the elimination of holy war challenges believers to live up to the highest tenets of their respective faiths.