Battle of the Zab, 750 AD.

 Marwan II (744–50), the grandson of Marwan I, led an army into Damascus in December 744, where he was proclaimed caliph. Moving the capital to Harran a rebellion soon broke out in Syria resulting in retaliatory action against the cities of Homs and Damascus (745). Further opposition broke out in Iraq and Iran from the Kharijites (746) who brought forward their claimants to the caliph. No sooner was the revolt suppressed when a time a more serious threat had arisen in Khorasan.

Around 746, Abu Muslim assumed leadership of the Hashimiyya in Khurasan. In 747, Abu Muslim successfully initiated an open revolt in Khurasan against Umayyad rule. With an army he gained control over the province of Khurasan and in 749 captured Kufa, the last Umayyad stronghold in Iraq. In November of the same year Abul Abbas as-Saffah was recognized as the new caliph.

From his capital in Harran, Marwan II mobilized his troops and advanced toward Iraq. In January 750 the two forces met along the banks of the Zab River.

The battlefield and troop strength

One source {1} sets the date of the battle at January 25, 750 AD, but little else is known about its location other than it was fought along its banks. If this is so, then the river might be considered paltry allowing troops to freely cross at any point, alternatively, despite a low water level, its banks could still pose a problem forcing a constricted passage. Some sources make reference to the construction of a pontoon bridge to help speed the crossing by Umayyad troops {2}, nonetheless, troops were able to cross at other points of the river

The photos seen here are sourced from Wiki Commons and are placed merely to give a general impression of the river and nearby terrain. 

Troop strengths vary from modest to exaggerated, but all sources give the Umayyad a numerical advantage. However, that advantage was offset with some Umayyad troops having questionable morale brought about through past uprisings and defeats plus the relocation of the capital. For this game we shall dispense with the calculation of numbers and give both sides two commands each (24 elements).

Tomorrow, the battle.

{1} The Battle of Zab, Cohn and Russell.

{2} The Armies of the Caliphs, Hugh Kennedy.