The Imams of Yemen and later also the Kings of Yemen were religiously consecrated leaders belonging to the Zaidiyyah branch of Shia Islam. They established a blend of religious and political rule in parts of Yemen from 897. Their imamate endured under varying circumstances until the republican revolution in 1962, then the formal abolition of the monarchy in 1970. Zaidiyyah theology differed from Ismailis or Twelver Shi'ites by stressing the presence of an active and visible imam as leader. The imam was expected to be knowledgeable in religious sciences, and to prove himself a worthy headman of the community, even in battle if this was necessary. A claimant of the imamate would proclaim a "call" (da'wa), and there were not infrequently more than one claimant. The historian Ibn Khaldun (d. 1406) mentions the clan that usually provided the imams as the Banu Rassi or Rassids. In the original Arab sources the term Rassids is otherwise hardly used; in Western literature it usually refers to the Imams of the medieval period, up to the 16th century. The Rassid branch that came to power with imam al-Mansur al-Qasim (r. 1597-1620) is known as Qasimids (Al al-Qasimi).