(May–July 657), series of negotiations and skirmishes during the first Muslim civil war (fitnah; (fitnah) 656–661), ending in the arbitration of Adhruḥ (February 658–January 659), which undermined the authority of Alīʿ as fourth caliph and prepared for establishment of the Umayyad Dynasty.

Muʿāwiyah (Muʿāwiyah I), governor of Syria, refused to recognize ʿAlī as the new caliph, calling instead for vengeance for the blood of his murdered kinsman, the third caliph, ʿUthmān. ʿAlī responded by invading Syria. The two armies met along the Euphrates River at Ṣiffīn (near the Syrian-Iraqi border), where they engaged in an indecisive succession of skirmishes, truces, and battles, culminating in the legendary appearance of Muʿāwiyah's troops with copies of the Qurʾān impaled on their lances—supposedly a sign to let God's word decide the conflict. ʿAlī delegated Abū Mūsā al-Ashʿarī as his representative, while Muʿāwiyah sent Amr ibn al-ʿĀṣʿ. The two men, on the basis of the Qurʾān and the traditions (Ḥadīth) of the Prophet and in the presence of witnesses, were to decide whether ʿUthmān had been guilty of abusing the divine law. If he had sinned and his murder was justified, then ʿAlī's position as caliph would be secure; a verdict of innocence, however, would justify Muʿāwiyah's attempts at vengeance and dislodge ʿAlī. In a meeting at Adhruḥ (in present Jordan; some suggest Dūmat al-Jandal, in present Saudi Arabia) in February 658, the arbitrators decided on ʿUthmān's innocence. ʿAlī immediately denounced the decision as invalid and reneged on his oath to be bound by the arbitration; Muʿāwiyah, meanwhile, was proclaimed caliph by some of his Syrian supporters. In January 659 the arbitrators met at Adhruḥ, formally deposed both ʿAlī and Muʿāwiyah, then discussed the candidacy of ʿAbd Allāh ibn ʿUmar and Muʿāwiyah for the caliphate; no decision was reached.

ʿAlī and Muʿāwiyah retained their own partisans, but, as Muʿāwiyah's authority began to expand into Iraq and the Hejaz (western Saudi Arabia), ʿAlī's diminished to Kūfah, his capital. With ʿAlī's assassination in 661, Muʿāwiyah was free to establish himself as the first caliph of the Umayyad house.