When
Umar ibn al-Khattab became caliph in 634, the Muslim conquests outside the
Arabian Peninsula increased, and Al-Ahnaf encouraged his people to participate. When
Persians surrounded a Muslim division led by
al-'Alaa ibn al-Hadrami, in
Iraq, Umar ordered
Utbah ibn Ghazwan to go to their rescue. Utbah sent a force of 12,000, most of them form the
Banu Tamim, and among them was al-Ahnaf who played a major role in breaking the siege. In 639, al-Ahnaf returned to
Medina bringing news of the conquest of
Tustar, and informing
Umar of affairs in
Persia. After listening to him, Umar said; Al-Ahnaf replied: Umar gave him permission to pursue
Sasanian king
Yazdgerd III, and put al-Ahnaf at the head of a Muslim army to complete the conquest of
Khurasan in the last stages of its annexation. Al-Ahnaf followed
Yazdgerd III until he barricaded himself in the town of Marwir-Rawdh.
Yazdgerd III unsuccessfully appealed to neighboring
Turkic and
Chinese rulers asking for help. After sending for reinforcement from
Kufa, Al-Ahnaf finally captured Marwir-Rawdah, completing the conquest of
Khurasan. Towns beyond Khurasan signed peace agreements with al-Ahnaf. He then sent a letter to
Umar informing him of the conquest, but added that
Yazdgerd III had escaped and fled to
Balkh. Umar forbade him to continue deeper into Persia, but ordered him to consolidate Islam in the conquered lands. Al-Ahnaf obeyed the orders but kept an eye on
Yazdgerd III‘s moves. When he later learned that
Turks joined Persians and were both approaching, he rallied his army and camped outside the town. With an army that was only a fraction of the enemy's, he managed to defeat the Persians in
Battle of Oxus river, killing their leader, while the Turks retreated to their land.
Yazdgerd III then fled eastward from one district to another, until at last he was killed by a local miller for his purse at
Merv,
Turkmenistan in 651. ==During and After the
Fitna==