In the spring of 37 AH ʿAli ﵁ set out from Kufa with an army that contemporaries estimated at seventy to a hundred thousand men. Muʿawiya ﵁ marched east from Damascus with a comparable force. The two armies maneuvered against each other for weeks on end through the Jazira, until they met on the plain of Siffin, on the western bank of the Euphrates, west of present-day Raqqa.
Muʿawiya ﵁'s army arrived first and occupied the only usable access to the river water on that stretch of plain: a shallow crossing where the bank sloped down and water buckets could be filled. Here he posted a heavy guard under Abu al-Aʿwar al-Sulami, with the express order to let ʿAli ﵁'s men draw not a single drop of water. This was more than a tactical detail. It was an attempt to force ʿAli ﵁'s army to choose between thirst and a frontal attack.
ʿAli ﵁ sent Saʿsaʿah ibn Suhan ﵁ as an envoy to Muʿawiya ﵁'s tent, with the request for free access to the water for all Muslims, as the Prophet ﷺ had commanded for access to wells and pastures. Amr ibn al-As ﵁ advised Muʿawiya ﵁ to grant the request, for strategic reasons. Muʿawiya ﵁ refused.
Saʿsaʿah ﵁: “ʿAli says: this is the water of Allah. We request our share, for our horses and our men.”
Muʿawiya ﵁'s envoy: “The answer is no. The sword will decide your share.”
ʿAli ﵁ appointed Malik al-Ashtar ﵁ to storm the water post with cavalry. The attack on the usable crossing was fought out in a short but fierce struggle. Abu al-Aʿwar’s guard was driven off. ʿAli ﵁ then himself had Abu al-Aʿwar’s men, after their retreat, offered leave to draw water from the Muslim bank, the very thing they had refused to offer his men. Tabari records this decision as one of the enduring images of Siffin.