In Syria, disorder and incitement to commotion continued unabated. Uthman's shirt, besmeared with his blood and the chopped-off fingers of his wife, Naila, were exhibited from the pulpit. In this manner, Muawiya raised the entire country of Syria against Ali. Ultimately, both the parties, opposed to each other, converged on Siffin where their armies pitched their camps in 37/657. Even at this stage, Ali sent three men, viz. Bashir bin Amr bin Mahz Ansari, Saeed bin Qais Hamdani, and Shis bin Rabiee Tamini to Muawiya to induce him to settle for union, accord and coming together. According to Tabari (5h vol., p. 243), Muawiya replied that, "Go away from here, only the sword will decide between us."
With an army of some 80,000 strong, mainly recruited from Iraq, Ali set out from Kufa, planning to march through upper part of Iraq and invade Syria from the north. Ali, then pushed on to Raqa, on the left bank of the Euphrates. Here his troops came across the Syrian vanguard but it withdrew without engagement. The next problem was how to cross the river. Ali wanted to construct a bridge of boats, but the people of Raqa were hostile. It was only after Ali's general, Ashtar, had threatened them with death that they consented to help in building the bridge which was completed under the great difficulties. Ali's men then advanced along the right bank of the river in the direction of Aleppo. At Sur-Rum they had a brief skirmish with a Syrian outpost before they reached the plain of Siffin, where they found Muawiya's forces drawn up in strength and waiting for them.
Ali soon discovered that the Syrian positions controlled the water supply of the whole valley, and that there was no access to the river for his men. Muawiya obviously intended to use thirst to drive Ali's men to surrender. Muawiya had, however, underestimated the calibre of Ali's troops. Ali, however wrote a letter to Muawiya, which reads: "You have fore-stalled me in pitching the stables for the horses of your cavalry. Before I could declare war on you, you have declared war on us. It was bad move on your part to cut off our supply of water. It behoves you to allow us the natural supply of water. Failing this, we will be reluctantly forced to fight with you." On receiving this letter, Muawiya conferred with his advisers, who urged him not to yield up the advantage he had gained. Ali was therefore left with no alternative but to attack at full gallop and inflicted a crushing defeat on Muawiya's forces, and took charge of water supply. Now it was the turn of Ali's counsellors to urge control of the water supplies and for the soldiers of Muawiya to suffer the rigours of extreme thirst. But Ali ordered his men to allow the Syrians free access to the river, saying: "Our religion and ethical code does not permit us to stop water supply, and so pay our enemy back in his own coin. I do not want to follow the way of the ignorant people."
Ali's next step was, as usual, to try and come to a peaceful settlement. He deputed Bashir bin Amr, Sa'id bin Qais and Shabus bin Rabi, but Muawiya declined the offer to the delegates. Ali still did not give up hope but a second delegation, consisting of Adi bin Hatim, Yazid bin Qais, Ziyad bin Hufza and Shabis bin Rad, also failed to persuade Muawiya to come to an amicable settlement. For the next three months, Zilhaja, Muharram and Safar 36/May, June and July, 657, the armies remained in camps at Siffin, facing each other neither at war nor at peace. This period of negotiations lasted 110 days, during which time, the Arabian chroniclers maintain that Ali made as many overtures for peace as there were days. There was much heat in the discussions with Muawiya, and finally Ali was obliged once again to resort to arms on 8th Safar, 36/July 26, 657.
Historian Yaqubi (2nd vol., p. 188) writes that Ali had 80,000 men, including 70 Companions who participated in Badr, 70 Companions who took oath at Hudaibia, and 400 prominent Ansars and Muhajirs; while Muawiya had 1,20,000 Syrians.
During the 110 days of negotiations, no fewer than 90 skirmishes were fought. Almost every day one tribal column would engage an enemy in combat, sometimes two or more engagements would be fought in one day. Heart-broken at the amount of Muslim blood that had already been shed in vain, Ali made one last bid for peace with Muawiya, at the start of the new year, but of no avail. At long last, Ali decided on a general engagement, and thus the battle of Siffin broke out on 8th Safar, 36/July 26, 657. A fierce battle was fought between them on the whole day, and it even continued in the darkness of that night, which is known as laila'tul harir (the night of clangour). William Muir writes in "The Caliphate, its Rise and Fall"(London, 1924, p. 261) that, "Both armies drawn out in entire array, fought till the shades of evening fell, neither having got the better. The following morning, the combat was renewed with great vigour. Ali posed himself in the centre with the flower of his troops from Medina, and the wings were formed, one of the warriors from Basra, the other of those from Kufa. Muawiya had a pavilion pitched on the field; and there, surrounded by five lines of his sworn body-guards, watched the day. Amr with a great weight of horse, bore down upon the Kufa wing which gave away; and Ali was exposed to imminent peril, both from thick showers of arrows and from close encounter. Reproaching the men of Kufa for their cowardice, the Caliph fought bravely, his unwieldy figure notwithstanding, sword in hand, and manfully withstood the charge. Ali's general Ashtar, at the head of 300 readers of (the Koran) led forward the other wing, which fell with fury on Muawiya's Turbaned body-guard. Four of its five ranks were cut to pieces, and Muawiya, bethinking himself of flight, had already called for his horse, when a martial couplet flashed in his mind, and he held his ground."
The following morning, the battle started up again. Edward Gibbon writes in "The Rise and Fall of the Roman Empire" (London, 1848, 3rd vol., p. 522) that, "The Caliph Ali displayed a superior character of valour and humanity. His troops were strictly enjoined to wait the first onset of the enemy, to spare their flying brethren, and to respect the bodies of the dead, and the chastity of the female captives. The ranks of the Syrians were broken by the charge of the hero, who was mounted on a piebald horse, and wielded with irresistable force, his ponderous and two edged sword."
Appalled by the carnage, Ali sent a message to Muawiya and challenged him to single combat, saying that whoever won should be the Caliph. In Gibbon's words, "Ali generously proposed to save the blood of the Muslims by a single combat; but his trembling rival declined the challenge as a sentence of inevitable death." Muawiya had indeed lost his nerve, and was about to flee from the field, a trick of his accomplice Amr bin al-A'as saved them from destruction.
At length, Muawiya made his mercenaries tie copies of Holy Koran to their lances and flags, demanding for the decision of arbitration. Tabari (6th vol., p. 46) writes that, "The defeat started Muawiya in the face. Amr bin al-A'as, however, had a trick up his sleeve for this emergency, and it was the raising of the Koran aloft on spear-heads, and announcing, "Brethren, this Book of God alone will decide between you and us." It will be recalled that even before the commencement of the battle, Ali had invited Muawiya by sending his three men to turn to the Koran for a decision, but his offer was declined by telling, "Go away from here, only the sword will decide between us." (Tabari, 5th vol., p. 243). And now they sought the intercession of the Holy Koran to escape the unpleasant consequences of an ignominious defeat. At this Ali came forward and expostulated his soldiers, saying, "It is an infamous stratagem and a nefarious device of Amr and Muawiya to cloak their defeat. Beware of the trick which they are playing. You should fight to a finish." But Ali's men refused to fight. Ali, with a great expectation of victory in sight, was therefore impelled to call a retreat.
Ali's supporters during the battle of Siffin were called ahel-i Iraq, or Shiat'i Ali, while his opponents became known as ahel-i Sham, or Shiat'i Uthman and Shiat'i Muawiya. But Ali called them al-kasitun (those who act wrong), a word derived from the Holy Koran that: "And as for the deviators, they shall be for the hell, a fuel." (72:15), wherein the word al-kasitun means the fuel of hell-fire.