In 644, al-'Ala' once again attacked Fars from Bahrain, reaching as far as Estakhr, until he was repulsed by the Persian governor (marzban) of Fars, Shahrag. Some time later, Uthman ibn Abi al-As managed to establish a military base at Tawwaj, and soon defeated and killed Shahrag near Rew-shahr.

In 648, 'Abd-Allah ibn al-'Ash'ari forced the governor of Estakhr, Mahak, to surrender the city. However, the inhabitants of the city would later rebel in 649/650 while its newly appointed governor, 'Abd-Allah ibn 'Amir, was trying to capture Gor. The military governor of Estakhr, 'Ubayd Allah ibn Ma'mar, was defeated and killed. In 650/651, Yazdegerd went there to plan an organized resistance against the Arabs, and, after some time, went to Gor. However, Estakhr failed to put up a strong resistance, and was soon sacked by the Arabs, who killed over 40,000 defenders. The Arabs then quickly seized Gor, Kazerun and Siraf, while Yazdegerd fled to Kerman. Muslim control of Fars remained shaky for a time, with several local rebellions following the conquest.