Following Muhammad's death in 632, Abu Bakr became the new leader of the Muslim community. After reasserting Muslim control over the dissident tribes of Arabia, he sent the Muslim armies against the empires of Byzantium and Sassanian Persia initiating a wave of conquests which were continued by his successor Umar (r. 634–644) bringing about almost complete collapse of the Sassanians, and restricting the Byzantine Empire to Anatolia and North Africa. The conquests brought Muslims bounteous revenue and lands. Particularly in Iraq, the former crown-lands and the lands of the Persian aristocracy were now in Muslim hands. These became communal property administered by the state. The revenue was distributed among the conquering armies, who had settled in Iraq. Umar also left the provincial administration to the respective governors, who ruled with considerable autonomy, and provincial surplus was spent on the settlers of the conquered territories instead of being sent to the capital.