In fact, the greatest tensions and worst violence tended to take place among people of the same faith. This was true on both a political and an ideological level. Surprising as it may sound, Muslim princes frequently allied with Christian kingdoms, and Christian kings with Muslim powers, against common enemies, whether Christian or Muslim. And holy war was waged among members of the two religions as often as between them. Latin (Catholic) Christians crusaded against Greek (Orthodox) Christians, while Sunni Muslim rulers proclaimed jihad against Shi'a Muslim rulers. Nor were Jews merely hapless bystanders in these conflicts. On the contrary, Jewish communities from Spain to Egypt were powerful and politically engaged; and Jews acted as governors, generals, warriors, and administrators in Christian and Muslim kingdoms. Like Christian and Islamic society, the Jewish world was one riven by brutal internal power struggles. All of this flies in the face of popularly established notions of the age of Crusade, and of the history of encounters among Christianity, Islam, and Judaism.
-Brian A. Catlos, Infidel Kings and Unholy Warriors: Faith, Power, and Violence in the Age of Crusade and Jihad