overt religious action, concludes that 6% of the wars listed in their encyclopedia can be labelled religious wars. Holy war concepts in religious traditions While early empires could be described as henotheistic, i.e. dominated by a single god of the ruling elite (as Marduk in the Babylonian empire, Assur in the Assyrian empire, etc.), or more directly by deifying the ruler in an imperial cult, the concept of "holy war" enters a new phase with the development of monotheism. Ancient warfare and polytheism During classical antiquity, the Greco-Roman world had a pantheon with particular attributes and interest areas. Ares personified war. While he received occasional sacrifice from armies going to war, there was only a very limited "cult of Ares". In Sparta, however, each company of youths sacrificed to Enyalios before engaging in ritual fighting at the Phoebaeum. Hans M. Barstad (2008) claimed that this ancient Greek attitude to war and religion differed from that of ancient Israel and Judah: 'Quite unlike what we find with the Greeks, holy war permeated ancient Israelite society.' Moreover, ever since the pioneering study of Manfred Weippert, "»Heiliger Krieg« in Israel und Assyrien" (1972), scholars have been comparing the holy war concept in the (monotheistic) Hebrew Bible with other (polytheistic) ancient Near Eastern war traditions, and found 'many [striking] similarities in phraseology and ideology'. Buddhism Christianity According to historian Edward Peters, before the 11th century, Christians had not developed a concept of holy war (bellum sacrum), whereby fighting itself might be considered a penitential and spiritually meritorious act. During the ninth and tenth centuries, multiple invasions occurred which led some regions to make their own armies to defend themselves and this slowly lead to the emergence of the Crusades, the concept of "holy war", and terminology such as "enemies of God" in the 11th century. In early Christianity, St. Augustine's concept of just war (bellum iustum) was widely accepted, but warfare was not regarded as a virtuous activity and expressions of concern for the salvation of those who killed enemies in battle, regardless of the cause for which they fought, was common. During the era of the Crusades, some of the Crusaders who fought in the name of God were recognized as the Milites Christi, the soldiers or the knights of Christ. The Crusades were a series of military campaigns against the Muslim Conquests that were waged from the end of the 11th century