After the battle of Jamal was over, Imam Ali (A.S.) returned from Basra to Kufa in Rajab of 36 A.H. He decided to transfer the capital of his government to Kufa from Madina because it was more centrally placed in the Muslim Empire, and he could halt Muawiya's progress into Iraq.
Before marching towards Muawiya, Imam Ali (A.S.) tried to settle matters peacefully by sending Jarir, chief of Bani Bajila and the governor of Hamdan, to Syria as an envoy. However, Jarir became so engrossed in the entertainment that Muawiya put his way, that he wasted his time in Syria. He finally returned three months later with the useless message that peace could only be negotiated if the murderers of Uthman were brought to justice. Malik al-Ashtar accused him of having wasted time in effeminate pleasures with Muawiya, who purposely kept him long enough to mature his plans of hostilities. Jarir left Kufa and joined Muawiya.
Imam Ali (A.S.) decided that matters could be only decided by war, so he marched without delay through the Mesopotamian desert to Riqqa at the banks of the Euphrates. After crossing the river by constructing a bridge they came across the Syrian outposts at Sur al-Rum. There were a few skirmishes between the armies but the Syrians gave way and in the month of Zilhaj of 36 A.H., the army of Imam Ali (A.S.) came into sight of Muawiya's main forces, which had already camped at Siffin.
At Siffin, Muawiya had stationed his general, Abul Awr, with 10,000 men on the river to stop the access to water for Imam Ali's (A.S.) army. Imam Ali (A.S.) sent Sasaa Bin Sauhan al-Abdi to Muawiya that this action was not necessary because, after all, the people whom he was refusing water were also Muslims. He further assured Muawiya that if the situation had been reversed, the river would have been open to both armies. However, Muawiya sent back a message that the murderers of Uthman had not allowed him any water when they had laid siege to his palace, and Muawiya was avenging that action.
Imam Ali (A.S.) knew that this situation would be intolerable and he launched an attack under Malike Ashtar. The brave commander secured the river after heavy fighting and Abul Awr was dislodged from its banks. Having control of the river, Imam Ali (A.S.) kept to his word and allowed unlimited access to Muawiya's side.
Imam Ali (A.S.) divided his army of 90,000 men into seven units each commanded by brave warriors. Muawiya similarly divided his army of 120,000 men into seven columns. Everyday one column from each army would engage one another in combat.
The battles were mostly restricted to single combats or small groups fighting because Imam Ali (A.S.) was trying to avoid the serious loss of Muslim lives that would have resulted from a full scale battle. The month of Zilhaj ended in this manner and the month of Muharram, in which fighting is forbidden, set in. During this month, Imam Ali (A.S.) tried hard to resolve the crisis by negotiation, but to no avail. He pointed out that he was ready to punish the murderers of Uthman if Muawiya would point them out. However, Muawiya did not wish the matter to end so easily, because it was the issue of Uthman's unavenged death that had enabled him to gather such a large army.
In the month of Safar fighting was resumed. For a week, fierce battles raged all day. Everyday the conflict got more severe and bitter. In the second week Imam Ali (A.S.) came to the battlefield for the first time. After a series of single combats, in which he overcame every opponent with his awesome skill, no body would come to fight him.
He was forced to disguise himself to get anybody to challenge him. On one such occasion, an unsuspecting warrior from Muawiya's side attacked Imam Ali (A.S.). The man was struck with a single sweep of Zulfiqar with such force by Imam Ali (A.S.) that the upper half of his body was severed from the lower half. Those who watched thought that the blow had missed, and it was only when the horse moved and the two halves fell to the ground, that people realized what had happened.
Day after day the loss of lives increased, especially in the ranks of Muawiya. However, Imam Ali (A.S.) also lost several distinguished Companions of the Holy Prophet (S.A.W.) from his side. Amongst them were Hashim bin Utba and Ammar Yasir.
Ammar, who was 93 years old, had been informed by the Holy Prophet (S.A.W.) that he would die fighting rebels and enemies of Islam. This was well known by all, and when he died there was some commotion in Muawiya's army. He managed to quieten them down by saying that, since Ammar had been brought to fight by Imam Ali (A.S.), it was he who was the cause of his death. He said that Imam Ali (A.S.) therefore was the rebel that the prophecy talked about, and not Muawiya. This incredible argument was accepted by his men and war continued until the 13th night.
On that day the commander-in-chief of Imam Ali's (A.S.) army, Malike Ashtar, attacked the enemy ferociously. His shout of Allahu Akbar, every time he killed a man, was heard no less than 400 times.
The hero of the battle began to bring on victory when Amr al-Aas on Muawiya's side said, "Call the enemy to the Word of God."
Muawiya eagerly accepted these words and his men raised 500 copies of the Holy Qur'an on their spears, saying that the Holy Book would decide their differences. This trick had a strange effect on some people in the army of Imam Ali (A.S.), who dropped their weapons and agreed that the Holy Qur'an should decide the matter.
Imam Ali (A.S.) stepped into the battlefield urging his men to continue fighting and ignore the tricks of Muawiya, but they disobeyed. The war thus came to an unsatisfactory end, and it was decided that one representative from each side should meet to reach a final decision.
Imam Ali (A.S.) wanted Abdullah bin Abbas or Malike Ashtar to represent him, but his men insisted that Abu Musa Ash'ari be chosen instead. Muawiya appointed Amr al-Aas to represent him. Abu Musa had neither wit nor tact and was no match for the cunning Amr al-Aas.
In the meeting that took place some months later, Abu Musa was badly tricked by Amr into giving up the rights of Imam Ali (A.S.), and it is as follows.
Decision of the umpires
The time for arbitration having come, the umpires proceeded to Dumat-al-Jondel or Azroh, each with a retinue of four hundred horsemen according to the agreement. Many a leading Chief from Mecca, Medina, Iraq and Syria went there to watch the proceedings, which were to decide the future of Islam. Abdallah bin Abbas, who accompanied Abu-Musa to preside at the daily prayers, while having a discourse with Abu-Musa upon the topic of arbitration, urged him to beware of the crafty ways of his astute colleague and to keep particularly in his mind the fact that Ali had no blemish to render him incapable of government, nor Muawiya any virtue to qualify him for it. When Abu-Musa reached Duma, Amr bin Aas received him with great respect. A private conference was held between the two alone in a pavilion erected for the purpose. Amr was already well aware of the weaknesses in Abu-Musa's character. He treated Abu-Musa with utmost respect and civility till he brought him completely under his influence. Having won his confidence, he made him admit that Osman was foully murdered. Then he asked him why the avenger of his blood, a near relation of his and an able administrator viz. Muawiya should not be taken as his successor. To this Abu-Musa replied that the succession should not be determined on such a basis which would give preference to Osman's Sons as legitimate claimants; but that they must above all things take care lest a mutiny should be kindled or civil wars break again. Upon this Amr bin Aas asked Abu-Musa to reject both Ali and Muawiya, and let the Faithful elect a third. This is the simplest and safest solution of the problem. 'I agree,' said Abu-Musa, let us go forth to pronounce. A tribunal was erected from which each of the umpires was to declare publicly his decision. Abu-Musa wished Amr to go up first, but Amr, alleging reasons to give preference to Ali's man, overcame all his scruples and insisted upon Abu-Musa going up first. Abu-Musa ascended and addressed the people thus: 'Brethren! I and Amr bin Aas, both of us, have given full consideration to the matter and have come to the conclusion that no other course to restore peace and to remove discord from the people can possibly be better than to depose both Ali and Muawiya in order that people may have their choice of a better man in their stead. I therefore depose both Ali and Muawiya from the Caliphate to which they pretend, in the manner as I draw this ring from my finger.' Having made this declaration Abu-Musa came down. Amr bin Aas now took his turn and went up to announce what he had to declare. 'You have heard,' he said, 'how Abu-Musa on his part has deposed his chief Ali; I, on my part, do depose him too and I invest my chief Muawiya with the Caliphate and I confirm him to it, as I put this ring upon my finger. I do this with justice because Muawiya is the avenger of Osman and his rightful successor.' So saying, he came down. This arbitration took place in the month of Ramadan, 37 A.H. or February 658 A.D.
Muawiya thus managed to escape certain defeat at Siffin. The damage done at the battle was great. Muawiya lost 45,000 men and 25,000 men were killed on the side of Imam Ali (A.S.).