Battle of Oxus River

The Battle of Oxus River was a significant battle in the 7th century, fought between the armies of the Sassanid Empire and the Muslim Arab army that had overrun Persia. Following his defeat, the last Sassanid Emperor, Yazdegerd III, became a hunted fugitive who fled to Central Asia and then to China.

Prelude
Khorasan was the second largest province of the Sassanid Persian Empire. It stretched from what is now present day north eastern Iran, Afghanistan and Turkmenistan. Its capital was Balkh, now in present day northern Afghanistan. In 651 the mission of conquering Khurasan was assigned to Ahnaf ibn Qais. Ahnaf marched from Kufa and took a short and less frequent route via Rayy and Nishapur. Rayy was already in Muslim hands and Nishapur surrendered without resistance. From Nishapur Ahnaf marched to Herat which in now in present day southern Afghanistan. Herat was a fortified town, Siege of Herat lasted for few months before Herat surrendered. With the surrender of Herat, the whole of the southern Khurasan came under Muslim control. With Herat under his firm control, Ahnaf marched north direct to Merv, in present Turkmenistan. Merv was the capital of Khurasan and here Yazdegerd III held his court. On hearing of the Muslim advance, Yazdegerd III left for Balkh. No resistance was offered at Merv, and the Muslims occupied the capital of Khurasan without a fight.

Battle
Ahnaf stayed at Merv and waited for reinforcement from Kufa. Meanwhile Yazdegerd had also gathered considerable power at Balkh and also sought alliance with the Khan of Farghana, who personally led the Turkish contingent to help Yazdegerd III. Umar ordered that Yazdegerd’s allied forces should be weakened by breaking up the alliance with the Turks. Ahnaf successfully broke up the alliance and the Khan of Farghana pulled back his forces realizing that fighting with the Muslims was not a good idea and might endanger his own kingdom. Yazdegerd's army was defeated at the Battle of the Oxus River and retreated across the Oxus to Transoxiana. Yazdegerd III had a narrow escape and fled to China. Balkh was occupied by the Muslims, and with this occupation the Persian war was over. The Muslims had now reached the outermost frontiers of Persia. Beyond that laid the lands of the Turks and still further laid China. The old mighty empire of the Sassanids had ceased to exist. Ahnaf returned to Merv and sent a detail report of operations to Umar and a historic letter that Umar was anxiously waiting for, the subject of which was that the Persian Empire had been conquered and a permission was sought whether Oxus should be crossed to invade Transoxiana or not. Umar ordered Ahnaf to consolidate his power south of Oxus.

Aftermath
Yazdegerd III after being defeated at the Battle of Oxus river was unable to raise another army and became a hunted fugitive. Following the battle he fled to Merv and was killed by a local miller for his purse. For many decades to come, this was the easternmost limit of complete Muslim rule.