Muslim commander Ahnaf ibn Qais marches north directly to Merv, in present Turkmenistan, where Yazdegerd III holds his court.
On hearing of the Muslim advance, Yazdegerd III leaves for Balkh.
No resistance is offered at Merv, and the Muslims occupy the capital of Khurasan without a fight.
Ahnaf stays at Merv and waits for reinforcements from Kufa.
Meanwhile, Yazdegerd has also gathered considerable power at Balkh and has sought alliance with the Khan of Farghana, who personally leads the Turkish contingent to help Yazdegerd III.
Uthman orders that Yazdegerd’s allied forces should be weakened by breaking up the alliance with the Turks.
Al-Ahnaf obeys the orders but keeps an eye on Yazdgerd III‘s moves.
When he later learns that the Turks have joined the Persians and both are approaching, he rallies his army and camps outside the town.
With an army that is only a fraction of the enemy's, he manages to defeat the Persians in Battle of Oxus river, killing their leader, while the Turks retreat to their land.
Yazdegerd's army retreats across the Oxus to Transoxiana; Yazdegerd himself has a narrow escape and fled to China.
Balkh is occupied by the Muslims, and with this occupation the Persian war is over.
The Muslims have now reached the outermost frontiers of Persia, beyond which lie the lands of the Turks and still further lies China.
The old and mighty empire of the Sassanids has ceased to exist.
Yazdgerd III then flees eastward from one district to another.
When Yazdegerd arrives in Merv, he demands tax from the Marzbān of Merv, losing also his support and making him an ally of Nezak Tarkan, the Hephthalite ruler of Badghis, who had helped him defeat Yazdegerd and his followers.
After his defeat, Yazdegerd is killed by a local miller for his purse while he is trying flee from Merv in 651.
