An important point on the differences in opinion between Shiia and Sunni Islam on the question of authority was brought up by Yitzhak Nakash in Foreign Affairs:
Unlike Sunnis, who in theory are expected to obey their rulers and even tolerate a tyrant in order to avoid civil strife and preserve the cohesion of the Muslim community, observant Shi’ites recognize no authority on earth except that of the imam. The twelfth imam is believed to be hidden from view and is expected to return one day as a messianic figure, the Mahdi. In his absence, there can be no human sovereign who is fully legitimate. This ambivalence toward worldly power has resulted in different interpretations within Shi’ite Islam regarding government accountability and the role of the clerics in state affairs. Khomeini’s concept of the rule of the jurist is only one among several competing views.
This point is important to understand when reading the differences between the anti-establishment movements in Iran and the religious extremist opposition to Sunni rule in Saudi Arabia and throughout the Arab Middle East. It is also important in understanding why democracy can work in Iraq.