What really happened at the battle of Siffin?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

First of all, dear brother, you should know that the doctrine of Ahlus-Sunnah wal-Jamaʻah is to refrain from arguing about what happened among the Companions of the Prophet ; to ask Allaah to be pleased with them all; and to believe that they were diligent in seeking the truth. Whoever among them made ijtihaad (personal reasoning) and was right will get a double reward, and whoever made ijtihaad and was wrong will get a single reward. One should not believe the fabricated false accounts in history books that undermine the Companions and depict what occurred between them as personal dispute or competition over worldly gains.

The battle of Siffeen took place between the army of ʻAli and the army of Muʻaawiyah both, due to the dispute over the unavenged murder of ʻUthmaan . Muʻaawiyah and his supporters demanded the immediate killing of the assassins of ʻUthmaan, whereas ʻAli demanded giving priority to uniting behind one Imaam (leader) and then investigating the murder of ʻUthmaan. ʻAli was not able to execute revenge on the murderers of ʻUthmaan because they were not identified (nobody knew who exactly the assassin was or who dealt the fatal blow). Moreover, there were many followers of the Khaawarij (who killed ʻUthmaan) among the forces of ʻAli, and they were ready to take up arms and fight. Their number amounted to 2000 warriors according to some narrations. Some of them left Madeenah to other countries after Muslims swore allegiance to ʻAli. Muʻaawiyah did not compete with 'Ali to be the Caliph; rather, he knew that ʻAli was more entitled to be Caliph than him. He only demanded retaliation for the murder of his kinsman ʻUthmaan.

In his book Al-Bidaayah wa An-Nihaayah, Ibn Katheer said:

"It has been reported through several chains of narration that Abu Muslim Al-Khawlaani and a group of men with him went to Muʻaawiyah and asked him, 'Do you rebel against ʻAli or are you equal to him?' Muʻaawiyah answered, 'By Allaah, I know that he is better than me and he has more right than me to rule, but do you not know that ʻUthmaan was killed unjustly? I am his cousin and I seek revenge for his blood. So, go to ʻAli and tell him to hand over ʻUthmaan’s murderers to me and (in return) I will let him be...'" [Al-Bidaayah wa An-Nihaayah]

It is sufficient for you, dear brother, to know that the Prophet described these two warring parties as "two great groups of Muslims."

It has been narrated on the authority of Abu Bakrah that he said, "I saw the Messenger of Allaah on the pulpit addressing (the people), and Al-Hasan ibn ʻAli (his grandson) was next to him. He would turn to the people then turn to him (Al-Hasan), and say, 'This son of mine is a leader, and Allaah may make peace between two great groups of Muslims through him.'" [Al-Bukhari]

Shaykhul-Islam Ibn Taymiyyah wrote, "This happened as he said it would, about thirty years after the death of the Prophet in the year 40 AH, when Allaah brought peace at the hands of Al-Hasan between the two great groups of Muslims who were at war at Siffeen, the forces of ʻAli and those of Muʻaawiyah."

Al-Haafith Ibn Hajar wrote about the creed of Ahlus-Sunnah Wal-Jamaaʻah pertaining to the fighting that took place between the Companions, "They unanimously agreed that not a single one of those (Companions) is to be dispraised; rather, they say, 'They made ijtihaad and were mistaken.'" He also said, "Ahlus-Sunnah Wal-Jamaaʻah unanimously agreed that it is obligatory to forbid the slandering of any of the Companions because of what happened between them, even if one knows which of them was right, because they did not fight except on the basis of what they thought was right (ijtihaad), (and Allaah forgives the one who is mistaken in his ijtihaad)."

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Allaah knows best.