[3] It grew to involve slaves and freemen, including both Eastern Africans and Arabs, from several regions of the Caliphate, and claimed tens of thousands of lives before it was fully defeated.

[4] Modern scholars have characterized the conflict as being "one of the bloodiest and most destructive rebellions which the history of Western Asia records,"[5] while at the same time praising its coverage as being among the "most fully and extensively described campaign[s] in the whole of early Islamic historical writing.

Throughout the 860s the various factions in the capital were distracted by this conflict, resulting in the deaths of several caliphs, army commanders and bureaucrats; the outbreak of multiple troop riots; a damaging civil war in 865–866; a great rebellion of the Kharijites in 866; and the virtual bankruptcy of the government.

This continuing instability greatly facilitated the initial success of the Zanj revolt, as the government proved incapable of committing sufficient troops and resources to subdue the rebels.

[12] Regardless of his origins, Ali appears to have spent at least a part of his youth living in the area of Rayy, and at an unspecified date he moved to the Abbasid capital of Samarra, where he mixed with some of the influential slaves of Caliph al-Muntasir (r. 861–862).

They became adept at raiding towns, villages and enemy camps (often at night), seizing weapons, horses, food and captives and freeing fellow slaves, and burning the rest to cinders to delay retaliation.

[21] As the rebellion grew in strength, they also constructed fortresses, built up a navy for traversing the canals and rivers of the region, collected taxes in territories under their control, and minted their own coins.

Early efforts by the Abbasid government to crush the revolt proved ineffectual, and several towns and villages were occupied or sacked, including al-Ubulla in 870 and Suq al-Ahwaz in 871.

A retaliatory campaign undertaken by the caliphal regent Abu Ahmad ibn al-Mutawakkil (known by his honorific of al-Muwaffaq) against the rebels in 872 ended in failure, and the Zanj remained on the offensive over the next several years.

[23] The continuing inability of the Abbasid army to suppress the revolt, caused in part by its preoccupation with fighting against the Saffarid Ya'qub ibn al-Layth's advance into al-Ahwaz and Iraq, eventually encouraged the Zanj to expand their activities to the north.

Al-Muwaffaq himself joined the offensive in the following year, and over the next several months the government forces succeeded in clearing the rebels out of the districts of Iraq and al-Ahwaz and driving them back toward their "capital" of al-Mukhtarah, to the south of Basra.

[25] Al-Mukhtarah was placed under siege in February 881, and over the next two and half years a policy by al-Muwaffaq of offering generous terms to anyone that voluntarily submitted convinced many of the rebels to abandon the struggle.

[26] Al-Masudi reported a "moderate" estimate of 500,000 casualties – though he added a clarification that this was "empty conjecture - rigorous calculation [of the number slain] is impossible" – and separately noted that 300,000 were killed at the Battle of Basra.

Sources of the revolt describe burnt cities and towns, the seizure of food and other resources by advancing armies, the abandonment of lands and the cessation of agricultural activity, disruptions in the regional trade, and the damaging of bridges and canals in the name of military exigency.

[33] The long term effects of the revolt, on the other hand, are more difficult to ascertain and opinions by modern historians vary; some like Bernard Lewis believe that the rebellion resulted in no significant changes, while others such as Theodor Nöldeke argue that the regions devastated by the conflict never fully recovered afterward.

[34] The significant arms and resources that the Abbasid government was required to throw against the Zanj meant that it was forced to divert its attention from other fronts for the duration of the conflict, resulting in the effective loss of several provinces.

The revolt may have also affected the government's ability to defend against the Byzantines, who scored several successes on the Anatolian frontier during this period, and possibly even indirectly contributed to the rise of the Qarmatians of Bahrain a few years later.