Battle of Naharwan
This battle has some unique significance attach to it. Imam Ali (A.s) eventually fought all the battles against Muslims during his reign, apart from this bitter reality this battle was against Kharijites. While Hazrat Ali A.S made preparations to attack Muawiya, the Kharijites from all over the country collected at Naharwan and began to create problems. This was on of the 3rd mainstream battle of Imam's life. However Kharijites were the people who stood against Imam and Muawiya in Siffen. Majorly they were the ex-followers of Imam but Muawiyas trick dismantled Imam's army and Kharijites were eventually born.
After the unsatisfactory conclusion to the battle of Siffin, Imam Ali ibn Abi Talib 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.
This group, the Kharijites, was furious at the way things had ended at Siffin. Although they had put down their weapons on the battlefield, demanding that Imam Ali accept the arbitration proposed by Muawiya, they now, however, held that Imam Ali had betrayed Islam by agreeing to the truce and should have referred judgment to the Qur'an alone or continued to fight. They demanded that he repent for this great sin.
On reaching their stronghold Imam Ali sent a messenger to the Kharijites demanding that those people who had murdered innocent Muslims around their camp should be surrendered. The Kharijites replied that they were all equally responsible for killing these sinners.
There was some reluctance amongst Imam Ali's army to fight the Kharijites, because these had been their companions against Muawiyah at the battle of Siffin. Imam Ali himself did not desire the bloodshed of these misguided fanatics, so he sent Abu Ayub Ansari with an offer of amnesty. Abu Ayyub al-Ansari declared "Whoever comes under this banner or separates from that party and goes to Kufa or al-Mada'in would get amnesty and he would not be questioned." Subsequently Farwah ibn Nawfal al-Ashja'i said that he did not know why they were at war with the Imam, and he separated along with five hundred men. Similarly group after group began to defect---some of them joining Imam Ali; eventually, only a core force of 1,800 die-hards were left under the command of Abdul Ibn Wahab. These Kharijites swore that they would fight Imam Ali ibn Abi Talib at any cost.
The Kharijites was named Rashidun army. The Kharijites fought deperatley but were outnumbered by the forces of Imam. Imam inflicted a crushing defeat on them. Out of four thousand Kharijites only a few dozen managed to escape; the rest were killed. Those who escaped bowed to take revenge. The victory proved to be a hollow one because it diverted the attention of Imam from his main struggle against Muahwiya.
The battle took place on the 9th Safar, 38 A.H. Two years later, in 40 A.H., Muawiya sent out three Kharijite assassins to kill Ali ibn Abi Talib, Muawiyah and his negotiator in the Arbitration, Amr ibn al Aas instead of making it look like only one was sent for Imam Ali. He and his childhood friend Amr knew of the plot and so those two survived but Imam Ali was assassinated by Abdul Rahman ibn Muljam's attack with a poisoned sword in the mosque of Kufa, on the 19th of the month of Ramadan, finally succumbing to poison on the 21st of the month of Ramadan.
In Nahaj ul Balagha Imam said : I am warning you that you will be killed on the bend of this canal and on the level of this low area while you will have no clear excuse before Allah nor any open authority with you. You have come out of your houses and then divine decree entangled you. I had advised you against this arbitration but you rejected my advice like adversaries and opponents till I turned my ideas in the direction of your wishes. You are a group whose heads are devoid of wit and intelligence. May you have no father! (Allah's woe be to you!) I have not put you in any calamity nor wished you harm.