A religious war or holy war is a war primarily caused or justified by differences in religion. The account of the conquest of Canaan by the Israelites in the Book of Joshua, the Muslim conquests of the 7th and 8th centuries, and the Christian Crusades (11th to 13th centuries) and Wars of Religion (16th and 17th centuries) are the classic examples but a religious aspect has been part of warfare as early as the battles of the Mesopotamian city-states.