After the unsatisfactory conclusion to the Battle of Siffin, Imam Ali (A) returned with his army back to Kufa on the 13th of Safar 37 A.H. During the march, a group of 12,000 men kept themselves at a distance from the main part of the army.
The group was furious at the way things had ended at Siffin. These were the Kharjites (Kharjite means one who rebels against religion). They were the same people who had put down their weapons on the battlefield. Now they said that Imam Ali (A) had betrayed Islam by agreeing to the truce and should have referred judgement to the Holy Qur'an alone or continued to fight. They demanded that he repent for this great sin.
When the army neared Kufa, the Kharjites camped at a village named Harura. They started saying that all Muslims were equal and nobody could rule over the other. In this way, they denounced both Imam Ali (A) and Muawiya and said that their belief was in "La Hukma Illa Lillah", meaning, "No Rulership except by Allah alone."
Imam Ali (A) went to their camp and tried to explain to them that they were misunderstanding the words "La Hukma Illa Lillah", and that in accepting the arbitration (peace talks) at Siffin, he had not gone against the teachings of the Holy Qur'an.
He pointed out that they themselves were at fault, because they should never have laid down their arms and forced him to call back Malike Ashtar, who was at the point of securing victory. He reminded them that they had pressed for the arbitration and had forced him to appoint Abu Musa Ash'ari as their representative. He told them that he found their present behaviour very strange, considering their involvement in Siffin. To this they admitted that they had sinned but now they had repented for it and he should do the same.
Imam Ali (A) replied that he was a true believer and did not have to repent because he had not committed any sin.
The Kharjites refused to accept the words of Imam Ali (A) and awaited the decision of Amr al-Aas and Abu Musa Ash'ari. When they learnt of the decision they decided to revolt, and they set up their headquarters at Nahrawan, a few miles from Baghdad. Some people came from Basra to join the rebels.
Meanwhile, Imam Ali (A) received news that trouble was brewing in Nahrawan. He was involved in raising another army to march against Muawiya and wrote to the Kharjites that it was high time they joined his army. However, the Kharjites insultingly wrote back that they would think about it when he repented for his mistake at Siffin.
Imam Ali (A) had already started towards Muawiya when he received the news that the Kharjites had raided the town of Mada'in but had been repelled back to their camp. They were now committing horrible crimes around Nahrawan and killing all those who did not accept their viewpoint.
There was a danger that the Kharjites might attack Kufa while Imam Ali (A) and his men were marching towards Muawiya, so Imam Ali (A) decided to stop them. He changed his course eastward, crossed the river Tigris and approached Nahrawan.
Imam Ali (A) sent a messenger to the Kharjites demanding that those people who had murdered innocent Muslims around their camp should be surrendered. The Kharjites replied that they were all equally responsible for killing these sinners.
There was some reluctance in the army of Imam Ali (A) to fight the Kharjites, because they had been their companions against Muawiya at Siffin. Imam Ali (A) himself did not desire the bloodshed of these misguided fanatics, so he placed his battle standard outside their camp and announced that all those who gathered around it or returned to their homes would be safe.
This announcement had the desired effect and most of the Kharjites began to leave. In the end, only a core of 1,800 die-hards were left under the command of Abdallah bin Wahab. These Kharjites swore that they would fight Imam Ali (A) at any cost.
The Kharjites attacked Imam Ali's (A) army with desperate courage. However, they did not stand a chance against the superior army that faced them and they were all killed except nine men. These nine managed to flee to Basra and elsewhere, where they spread the fire of their hatred and recruited more followers. Three years later, in 40 A.H., it was the Kharjites who sent out three assassins to kill Imam Ali (A), Muawiya and Amr al-Aas. The latter two survived but Imam Ali (A) was martyred following Ibne Muljam's cowardly attack in the mosque of Kufa.
Having disposed of the Kharjites at Nahrawan, Imam Ali (A) resumed his march to Syria. However, the chiefs of his followers urged him to stop at Kufa to let the men rest before the long journey and to enable the army to repair their weapons and armours. Imam Ali (A) agreed to this request and camped at Nukhayla outside Kufa. The soldiers were allowed to leave the camp for a day.
On the next day, hardly any men returned and at length, Imam Ali (A) entered Kufa and gave a stern sermon to the people. However, nobody came forward and finally, Imam Ali (A) turned away from them in disappointment. The Syrian expedition was abandoned, never to be resumed.
(Courtesy of:- Shia Ithna asheri Madressa)