'The Umayyad caliphs, and more so their successors, the Abbasids (750-1258), who ruled from Baghdad, imprisoned and killed Shia imams and encouraged Sunni ulama to define Sunni orthodoxy and contain the people of Shiism. By the tenth century Sunni jurists of the Hanbali school, known for their intolerance of Shiism, held sway over Baghdad, and fear of Shia revolts supported their penchant for purifying Islam. The last decades of that century witnessed anti-Shia violence in Baghdad and its environs – mosques and Ashouras were attacked, and Shias were even killed or burned alive. When in 971 C.E. Roman forces attacked the Abbasid domain, the first response of the caliph’s forces and the angry and terrified Sunnis was to blame the Shia. Shia houses in al-Karkh (in today’s Iraq) – which had become a refuge for Shias who escaped persecution in Baghdad – were torched as the attackers chanted, “You [Shias] are the cause of all evil.” In a pattern of behavior that would be repeated throughout the centuries down to the present, the Shia bore the brunt of popular frustrations with the failures of Sunni rulers. Treated as the enemy within, they were the first to come under suspicion when there was an external threat to the ruling Sunni establishment. By the middle of the eleventh century, persecuting the Shia of al-Karkh had become a custom; ever Saturday, Sunni mobs would show up at Shia mosques and shrines before looting the town, saying “You blasphemers! Convert to Islam!"
By the eleventh century these attitudes had also been canonized by Hanbali jurists, who condemned Shias as rafidis, or rejecters of the Truth. They said that Shias should not lead prayers or marry Sunnis, and that any meat that Shias slaughtered was not halal (permissible) for Sunni consumption. In short, the Shia were not to be treated as Muslims. After the Mongol sack of Baghdad and the destruction of the Abasid caliphate in 1258, attacks on Shiism grew even sharper. Hanbali characterization has in recent history found a reflection in the extremist Sunnis' demonization of Shiism, which regards the faith as a heresy and a bigger threat to "true" Islam than Christianity and Judaism.'
--Vali Nasr, The Shia Revival