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 southern Arabs or 'Yemenis' (), 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 the reign of Caliph Hisham ibn Abd al-Malik (), the Umayyad government appointed Mudaris as governors in Khurasan, 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 Khurasan and should have been the clear choice for governor. His Yemeni roots (he was the leader of the Azd in Khurasan), however, made him unpalatable to the caliph. Nasr on the other hand, in addition to his other qualities, was a Mudari and married to a Tamimi wife. He would therefore be acceptable to the numerous Mudari element of the Khurasani army, which outnumbered the Yemenis, but could also, as a local, help to reduce the Khurasani Arabs' discontent towards the Syria-centric Umayyad government. Nasr's own relatively obscure tribal background—from a non-noble family of the Layth tribe from Kinanah—also suited the caliph's purposes, as it meant that he lacked any local power base of his own. Indeed, Nasr's rule throughout his tenure was not fully accepted by many Arab tribesmen: aside from the Yemenis, who favoured their own candidate al-Kirmani and resented the shift in power back towards the Mudaris, the Qays around Nishapur refused to support him, and even the Syrian contingent sided with his opponents. Nasr was hence mostly reliant on the support of his wife's powerful Tamim tribe, living around Marv. As long as he was supported by a strong central government in Damascus, Nasr was able to keep his internal enemies in check, but in the troubles that followed Hisham's death in 743, that support vanished. In the event, Nasr would succeed in retaining his office for a decade,