until 738. Khalid in turn named Asad as governor of Khorasan. The two brothers thus became, according to Patricia Crone, "among the most prominent men of the Marwanid period". Asad's arrival in Khorasan found the province in peril: his predecessor, Muslim ibn Sa'id al-Kilabi, had just attempted a campaign against Ferghana and suffered a major defeat, the so-called "Day of Thirst", at the hands of the Turgesh Turks and the Soghdian principalities of Transoxiana that had risen up against Muslim rule. From the early days of the Muslim conquests, Arab armies were divided into regiments drawn from individual tribes or tribal confederations (butun or ‘asha‘ir). Despite the fact that many of these groupings were recent creations, created for reasons of military efficiency rather than any common ancestry, they soon developed a strong and distinct identity. by the beginning of the Umayyad period, this system progressed to the formation of ever-larger super-groupings, culminating in the two super-groups: the northern Arab Mudaris or Qaysis, and the south Arabs or "Yemenis" (Yaman), dominated by the Azd and Rabi'ah tribes. By the 8th century, this division had become firmly established across the Caliphate and was a source of constant internal instability, as the two groups formed in essence two rival political parties, jockeying for power and separated by a fierce hatred for each other. During Hisham ibn Abd al-Malik's reign, the Umayyad government appointed Mudaris as governors in Khorasan, except for Asad ibn Abdallah al-Qasri's tenure in 735–738. Nasr's appointment came four months after Asad's death. In the interim, the sources report variously that the province was run either by the Syrian general Ja'far ibn Hanzala al-Bahrani or by Asad's lieutenant Juday' al-Kirmani. At any rate, the sources agree that al-Kirmani stood at the time as the most prominent man in Khorasan and should have been the clear choice for governor. His Yemeni roots (he was the leader of the Azd in Khorasan), however, made him unpalatable to the Caliph. Abbasid era (750–861) Khorasan became the headquarters of the Abbasid Revolution against the Umayyads. It was led by Abu Muslim, who himself belonged to Khorasan. This province was part of the Iranian world that had been heavily colonised by Arab tribes following the Muslim conquest with the intent of replacing Umayyad dynasty which is proved to be successful under the sign of the Black Standard. Modern era Between the early 16th and early 18th