In 639 Umar established this encampment as a city with five districts, and appointed Abu Musa al-Ash'ari as its first governor. The city was built in a circular plan according to the Partho-Sasanian architecture. Abu Musa led the conquest of Khuzestan from 639 to 642 and was ordered by Umar to aid Uthman ibn Abi al-As, then fighting Iran from a new, more easterly "miṣr" at Tawwaj. In 650, the Rashidun Caliph Uthman reorganised the Persian frontier, installed ʿAbdullah ibn Amir as Basra's governor, and put the military's southern wing under Basra's control. Ibn Amir led his forces to their final victory over Yazdegerd III, the Sassanid King of Kings.