Arbitration
The two armies finally agreed to settle the matter of who should be Caliph by arbitration. The refusal of the largest bloc in Ali's army to fight was the decisive factor in his acceptance of the arbitration. The question as to whether the arbiter would represent Ali or the Kufans caused a further split in Ali's army. Ash'ath ibn Qays and some others rejected Ali's nominees, `Abd Allah ibn `Abbas and Malik al-Ashtar, and insisted on Abu Musa Ash'ari, who was opposed by Ali, since he had earlier prevented people from supporting him. Finally Ali was forced to accept Abu Musa.
Read more about this topic: First Fitna