to divide his army into three corps. Umar appointed Habib ibn Muslaima to capture Tiflis, Abdulrehman to march north against the mountains and Hudheifa to march against the southern mountains. With the success of all three missions, the advance into Armenia came to an end with the death of Umar in November 644. By then almost the whole of the South Caucasus was captured. Conquest of Khorasan Khorasan was the second-largest province of the Sasanian Empire. It stretched from what is now northeastern Iran, northwestern Afghanistan and southern Turkmenistan. In 651 the conquest of Khurasan was assigned to Ahnaf ibn Qais. Ahnaf marched from Kufa and took a short and less frequented route via Rey and Nishapur. Rey was already in Muslim hands and Nishapur surrendered without resistance. From Nishapur, Ahnaf marched to Herat in western Afghanistan. Herat was a fortified town, and the resulting siege lasted for a few months before it surrendered, bringing the whole of southern Khorasan under Muslim control. Ahnaf then marched north directly to Merv, in present-day Turkmenistan. Merv was the capital of Khurasan and here Yazdegred 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. Ahnaf stayed at Merv and waited for reinforcement from Kufa. Meanwhile, Yazdegerd had also gathered considerable power at Balkh and allied with the Turkic Khan of Farghana, who personally led the relief contingent. Umar ordered Ahnaf to break up the alliance. The Khan of Farghana, realizing that fighting against the Muslims might endanger his own kingdom, withdrew from the alliance and pulled back to Farghana. The remainder of Yazdegerd's army was defeated at the Battle of Oxus River and retreated across the Oxus to Transoxiana. Yazdegerd himself narrowly escaped to China.The Muslims had now reached the outermost frontiers of Persia. Beyond that lay the lands of the Turks and still further lay China. Ahnaf returned to Merv and sent a detailed report of his success to the anxiously-waiting Umar, and sought permission to cross the Oxus river and invade Transoxiana. Umar ordered Ahnaf to stand down and instead consolidate his power south of the Oxus. Persian rebellion and reconquest Umar was assassinated in November 644 by a Persian slave named Abu Lu'lu'a Firuz, after having refused to lift a tax imposed upon the latter. Soon after,