 To face these insurgencies, al-Ma 'mun turned to another of Tahir's sons, Abdallah ibn Tahir, to whom he entrusted the command of his army. Ibn Tahir first targeted Nasr ibn Shabath in northern Syria and the Jazira. Nasr was willing to acknowledge al-Ma 'mun's authority, but demanded concessions for his followers and remained hostile to the Abbasids'Persian officials, so that he had to be browbeaten into submission by a show of force before his capital, Kaysum, in 824 – 825. After securing his northern flank, Ibn Tahir marched through Syria into Egypt. There the two rivals, although not opposed in principle to al-Ma'mun as Caliph, were eager to maintain the status quo, and had already repulsed an invasion in 824 under Khalid ibn Yazid ibn Mazyad. Ibn Tahir however managed to outmanoeuvre both, so that Ali al-Jarawi quickly went over to him, leaving Ubayd Allah to submit and face deportation to Baghdad. In Alexandria, Ibn Tahir secured the departure of the Andalusians, who left the city for the Byzantine island of Crete, which they conquered and transformed into a Muslim emirate. On his return to Baghdad in 827, Abdallah ibn Tahir received a triumphal reception, and was appointed governor of Khurasan in 828, replacing Talha. His place in the west was taken over by al-Ma 'mun's younger brother Abu Ishaq al-Mu 'tasim. In Yemen, another Alid revolt broke out in 822 under Abd al-Rahman ibn Ahmad, but al-Ma'mun managed to secure his surrender by negotiations.
