China and India. In the Sasanian era, Khorasan was further divided into four smaller regions, and each region was ruled by a marzban. These four regions were Nishapur, Marv, Herat and Balkh. Khorasan in the east saw some conflict with the Hephthalites who became the new rulers in the area but the borders remained stable. Being the eastern parts of the Sassanids and further away from Arabia, Khorasan region was conquered after the remaining Persia. The last Sassanid king of Persia, Yazdgerd III, moved the throne to Khorasan following the Arab invasion in the western parts of the empire. After the assassination of the king, Khorasan was conquered by Arab Muslims in 647 AD. Like other provinces of Persia it became a province of the Umayyad Caliphate. Medieval era The first movement against the Arab conquest was led by Abu Muslim Khorasani between 747 and 750. Originally from Isfahan, scholars believe Abu Muslim was probably Persian. It's possible he may have been born a slave. According to the Ancient Persian historian Al-Shahrastani, he was a Kaysanite. This revolutionary Shi'a movement rejected the three Caliphs that had preceded Ali. Abu Muslim helped the Abbasids come to power but was later killed by Al-Mansur, an Abbasid Caliph. The first kingdom independent from Arab rule was established in Khorasan by Tahir Phoshanji in 821, but it seems that it was more a matter of political and territorial gain. Tahir had helped the Caliph subdue other nationalistic movements in other parts of Persia such as Maziar's movement in Tabaristan. Other major independent dynasties who ruled over Khorasan were the Saffarids from Zaranj (861–1003), Samanids from Bukhara (875–999), Ghaznavids from Ghazni (963–1167), Seljuqs (1037–1194), Khwarezmids (1077–1231), Ghurids (1149–1212), and Timurids (1370–1506). In 1221, Genghis Khan's son Tolui oversaw the Mongol subjugation of Khorasan, carrying out the task "with a thoroughness from which that region has never recovered." Rashidun era (651–661) Under Caliph Umar (), the Rashidun Caliphate seized nearly the entire Persia from the Sasanian Empire. However, the areas of Khorasan weren't conquered until during the caliphate of Uthman (). The Rashidun commanders Ahnaf ibn Qays and Abd Allah ibn Amir were assigned to lead the invasion of Khorasan. In late 651, the Rashidun army defeated the combined forces of the Sasanian and the First Turkic Khaganate in the Battle of the Oxus River. The next year, Ibn Amir concluded a peace treaty with Kanadbak,