The army of Marwan II fights a combined force of Abbasid, Shia, and Persian soldiers at the Zab on January 25, 750.

Marwan's army is far larger and seemingly more formidable than that of his opponents, as it contains many veterans of earlier Umayyad campaigns against the Empire; its support for the caliph, however, is only lukewarm.

The morale of the Umayyads has been damaged by the series of defeats inflicted earlier in the rebellion, while the morale of the Abbasid armies has increased.

The Abbasid army forms a spear wall, a tactic they had adopted from their Syrian opponents, presumably from witnessing it in earlier battles.

This entails standing in a battle line with their lances pointed at the enemy (similar to the stakes used by English longbowmen at Agincourt and Crécy many centuries later).

The Umayyad cavalry charges, possibly believing that with their experience they could break the spear wall.

This is a mistake on their part, however, and they are all but butchered.

The Umayyad army falls into retreat, its morale finally shattered.

Many are cut down by the zealous Abbasids or are drowned in the wintertime River Zab.

Over three hundred members of the Umayyad family die at this battle alone.

Marwan himself escapes the battlefield and flees down the Levant, pursued relentlessly by the Abbasids, who meet no serious resistance from the Syrians because the land has recently been laid waste by an earthquake and pestilence.

His heirs Ubaydallah and Abdallah escape to Ethiopia, where Ubaydallah will die in fighting there.