The immediate results of the battle of Nahāvend were disastrous to civilisation. Persia was traversed in all directions by bands of marauding Arabs, and the miserable inhabitants suffered as severely as they had suffered at the hands of the Mazdakites. “The Caliph Othman,”[80] writes Gibbon,[81] “promised the government of Khorāsān to the first general who should enter that large and populous country, the kingdom of the ancient Bactrians. The condition was accepted, the prize was deserved; the standard of Mahomet was planted on the walls of Herāt, Merou, and Balkh; and the successful leader neither halted nor reposed till his foaming cavalry had tasted the waters of the Oxus.” The ill-cemented power of the Caliph was more adapted for conquest than assimilation, and its area overrun by his undisciplined hordes was too vast to be held in permanent subjection. Conscious of their weakness, the Arabs spared no efforts to spread the tenets of Islām, which alone was capable of welding together communities differing widely in race, language, and customs. From this epoch dates the decline of the creed of Zoroaster throughout Persia and the countries of Central Asia. The assassination of the Caliph `Omar by a Persian slave was the signal for a general insurrection throughout this loosely knitted empire. This was not finally quelled till A.H. 31 (652), when Ibn `Āmir gained a victory over the Persians at Khwārazm on the Oxus, and compelled the country as far as Balkh to acknowledge the Caliph’s suzerainty.[82] In A.H. 41 (661) `Abdullah ibn `Āmir organised a successful expedition into Khorāsān and Sīstān;[83] and in the course of the following year Kays ibn al-Haytham was sent thither as provincial governor. He was superseded in A.H. 43 (663) by `Abdullah ibn Khāzim. In A.H. 45 (665) Ziyād, whom in the preceding year the Great Caliph Mo`awiya had officially recognised as his brother, was made governor of Basra and “the East.” Al-Hakam ibn `Āmir al-Ghifārī was sent in A.H. 47 (667) on an expedition into Khorāsān. He occupied Tokhāristān and the country south and south-east of Balkh as far as the Hindu Kush, and was, moreover, the first Arabian general to cross the Oxus.[84] Al-Hakam died at Merv in A.H. 50 (670), on his return from an expedition against the people of Mount Ashall.[85] In the following year Rabī` ibn Ziyād[86] el-Hārithī was sent to Khorāsān to succeed him.[87] About this date many Arabs migrated with their families to Khorāsān and settled there.[88] Rabī`’s first care was the reduction of Balkh, which had been the scene of a revolt, and this he effected without resorting to force. He also engaged the Turks in Kūhistān, and put them to rout. Among the fugitives was Nīzak Tarkhūn,[89] who perished later at the hands of Kutayba ibn Muslim. Rabī` also crossed the Oxus, but made no conquests on the farther side.[90] His death, and that of his master Ziyād, took place in A.H. 53 (673). He named his son `Abdullah as his successor, but the latter died two months later, and was succeeded by Khulayd ibn `Abdullah el-Hanafī. On the death of Ziyād the Caliph gave the governments of Kūfa, Basra, and Khorāsān to his own son `Ubaydullah, while he appointed Ziyād’s son `Ubaydullah, in supersession of Khulayd, as his lieutenant in Khorāsān. `Ubaydullah ibn Ziyād collected an army in Irāk, entered Khorāsān and, crossing the Oxus, penetrated into the mountains of Bokhārā,[91] and conquered Rāmtīna and half of Baykand. The Turks of Bokhārā were at that time governed by a princess named Khātūn, who acted as regent during the minority of her son Tughshāda. On the approach of the Arabs with an overwhelming force, Khātūn fled to Samarkand. According to Tabari,[92] so great was her haste that one of her shoes was left behind. It fell into the hands of the Arabs, and was valued by them at 200,000 direms.[93]