I am still more than happy with DS Wilson's explanation but this hypothesis is very intriguing:
--------snip----------What do you think?
Putting all of this together, it appears likely that the capacity for religious experience and the capacity for warfare have constituted a coevolutionary spiral that has intensified with the transitions from a hunting/gathering existence through subsistence agriculture to the evolution of the modern nation-state. As pointed out earlier, there is a correlation between the type of intergroup violence and the ecological context within which that violence occurs. Generally speaking, raiding/rustling is correlated with hunting/gathering and pastoralism, militia warfare with village agriculture, and professional warfare with urban society and the nation-state. There is a corresponding progression in the basic form of religious experience and practice: animism is most common among hunter-gatherers, while polytheism is more common among agriculturalists, and monotheism is most common in societies organized as nation-states. This is not to say there are no exceptions to this correlation. However, the fact that such a correlation can even be made points to the underlying ecological dynamics driving the evolution of subsistence patterns, patterns of warfare, and types of religious experience.