Al-Muʿtazz wrote to Muḥammad ibn ʿAbd Allah ibn Ṭāhir, inviting him to join his cause and reminding him of the pledges and covenants al-Mutawakkil had once exacted from him—namely, that the caliphate should pass to al-Muʿtazz after al-Muntaṣir. Muḥammad paid no heed; instead he replied with counter-arguments whose details are too lengthy to recount here.

Both al-Mustaʿin and al-Muʿtazz then dispatched letters to Mūsā ibn Bugha the Elder, who was stationed on the Syrian frontier fighting the people of Ḥimṣ, each summoning him to his own side. Along with their letters they sent him banners to bestow upon whomever he chose among his troops. Al-Mustaʿin further ordered him to leave a deputy in his post and march to Baghdad. Mūsā mounted in haste and rode ⦗1⦘ to Sāmarrāʾ, where he declared for al-Muʿtazz against al-Mustaʿin.

ʿAbd Allah ibn Bugha likewise slipped away from his father in Baghdad and made for Sāmarrāʾ, as did a number of other commanders and Turkic officers.

Al-Muʿtazz appointed his brother Abū Aḥmad ibn al-Mutawakkil to wage war on al-Mustaʿin and furnished him with an army. Abū Aḥmad set out with five thousand troops—Turks and others—toward Baghdad, prayed the Friday prayer in ʿUkbarā ⦗2⦗, and offered supplication on behalf of his brother al-Muʿtazz.

He reached Baghdad on the night of Sunday, the seventh of Ṣafar, and the armies massed there. A man called Bādhinjānah, who was in Abū Aḥmad’s camp ⦗3⦘, declaimed:

O sons of Ṭāhir, the armies of God have descended upon you,
and death is scattered among them far and wide;
Hosts led by Abū Aḥmad at their fore—
what an excellent patron, what an excellent ally!

Prolonged battles and truly dreadful disorders then broke out between the two sides; Ibn Jarīr ⦗4⦘ relates them at length.

Later al-Muʿtazz sent with Mūsā ibn Ashnās a reinforcement of three thousand men for his brother Abū Aḥmad. They arrived on the night preceding the last day of Rabīʿ al-Awwal, halting on the western bank at the Qutrubbul Gate, while Abū Aḥmad and his forces were positioned at the Shamāsiyyah Gate. The war blazed on, the fighting was fierce, and the slain lay everywhere.

Ibn Jarīr ⦗5⦘ reports that al-Muʿtazz wrote to his brother Abū Aḥmad, blaming him for slackening in the fight against the people of Baghdad. Abū Aḥmad replied:

The decrees of death have a roadway that reaches us,
and in Time we know both ease and constraint.