this period, especially following the Battle of Marj Rahit, that the longstanding Qays–Kalb split between the Arab tribes of Syria and the Jazira developed. It was paralleled in the division and rivalry between the Mudar, led by the Banu Tamim, and the Azd–Rabi'a alliance in Iraq and the eastern provinces. Together, these rivalries caused a realignment of tribal loyalties into two tribal confederations or "super-groups" across the caliphate: the "North Arab" or Qays/Mudar bloc, opposed by the "South Arabs" or Yemenis. These terms were political rather than strictly geographical, since the properly "northern" Rabi'a adhered to the "southern" Yemenis. The Umayyad caliphs tried to maintain a balance between the two groups, but their implacable rivalry became a fixture of the Arab world over the following decades. Even originally unaligned tribes were drawn to affiliate with one of the two super-groups. Their constant struggle for power and influence dominated the politics of the Umayyad caliphate, creating instability in the provinces, helping to foment the Third Fitna and contributing to the Umayyads' final fall at the hands of the Abbasids. The division persisted long after the Umayyads' fall; the historian Hugh Kennedy writes: "As late as the nineteenth century, battles were still being fought in Palestine between groups calling themselves Qays and Yaman". Sectarian and eschatological developments The death of Husayn produced widespread outcry and helped crystallize opposition to Yazid into an anti-Umayyad movement based on Alid aspirations. The Battle of Karbala contributed to the definitive break between what later became the Shi'a and Sunni denominations of Islam. This event catalyzed the transformation of Shi'ism, which hitherto had been a political stance, into a religious phenomenon. To this day it is commemorated each year by Shi'a Muslims on the Day of Ashura. This period also saw the end of purely Arab Shi'ism in the revolt of Mukhtar al-Thaqafi, who mobilized the marginalized and socioeconomically exploited mawali by redressing their grievances. Before then, non-Arab Muslims had not played any significant political role. Despite its immediate political failure, Mukhtar's movement was survived by the Kaysanites, a radical Shi'a sect, who introduced novel theological and eschatological concepts that influenced the later development of Shi'ism. The Abbasids exploited the underground network of Kaysanite propagandists during their revolution and the most numerous among their supporters were Shi'a and non-Arabs. The Second Fitna also gave rise to the idea of the Islamic Messiah, the Mahdi. Mukhtar applied the