The rebellion had five different stages we could say:
1. The preparation phase. In this phase, Ali ibn Muhammad al-Dibaj and a small group of men who whom he had met in his travelers entered the Sawadh in Southern Mesopotamia. At the time, according to al-Tabari, the primary agricultural occupations for the important cultivation and collection of crops in the region was performed by an amorphous group of slaves, the largest of whom would have come from Africa. Indeed however, these slaves came from diverse backgrounds, as we note in the words used by al-Tabari, various terms come to us, not only Zanj, implying a diversity in the slave population.
These slaves were employed in large scale cultivation, namely cutting and picking yields. They were in turn divided into hundreds of work teams, each with their own lieutenant who was a slave from among them that was explicitly of higher rank to the slaves under him by virtue of being a Muslim convert. These teams were in turn owned by certain estates ruled by various Arab clans who resided in the region. The oddity of this all, is that the slavery was new to the region and implies a serious demographic decline and economic change in the region. As during the preceding Sassanid period, we know the region did not require slave labor or imported populations for labour, but that is a side point.
Regardless, the Arab estates in the region seemed to own massive estates filled to the brim with slaves from across the Abbasid empire and nearby areas. Slaves were so abundant in this period that it was custom that at feasts, hosts would gift slaves free of charge to their guests and vice versa. All deals likewise were sealed by the gifting of slaves. Indeed, the warfare on all fronts of the Islamic world, produced large yields of slaves that were in turn due to reach the interior of the Abbasid world, Mesopotamia. Those who were not female or possessing great military prowess, seem to have been distributed into these large estates in the south which used them for labor to pick the fields. Their cost would have been low as they were packed into tiny hovels and communal houses where they would have slept upon the floor next to each other and wore only loin cloths and or moved about naked. Their food according to al-Tabari was usually surplus dates, fish from the swamplands or various types of fowl that were hunted and given as cheap sustenance. Their compliance to order was very loose, it would seem that the Arab overlords felt that the slaves would be maintained in order by implied threats and by simply being degraded in work. Considering their work, conditions and so forth, mortality may have been extremely high, both from disease, accident and over work in the fields (which due to the low cost of the slaves and their abundance, there was no compulsion or issue with overwork and hence killing slaves, they had little value). Additionally, there seems to have been a serious gender segregation in these estates. Most female slaves acting as domestic servants and concubines for their masters, producing children and working as slaves in the home. Men however were bunched into communities tightly packed into their work spaces away from women, implying that there was no intent by their masters to 'breed' the slaves or colonize the environment in a cooperative fashion alongside their slaves, as did Anglo-American slavery. It would seem that the mantra was: male slaves come and go.
During this, a certain Iranian man from northern Iran, born in the hill country, perhaps near Zanjan during the Khurramiyya rebellion named Ali ibn Muhammad al-Dibaj (a fake name, his real name is unknown, he never revealed it) arrived into the Sawadh. He had previously worked in the court in Baghdad as a poet but left alongside a group of followers, mostly Khwarij and Shi'a. He traveled to the Ahsa in Arabia and there attempted to create a rebellion, claiming to be the reincarnation of Ali and to be a god with the ability to create bolts of lightning. He was briskly rejected and sent off. He then spent a year with Bedouin in the Nejd where he apparently performed magic and claimed to control lightning and to be the reincarnation of various martyrs of the Shi'a resistance against the Caliphate, most importantly the so-called Shumat Imam of Arabia and Yahya ibn Umar of Karbala. Many Bedouin believed him and his magical powers, others were presumably Kharijite or Shi'a who saw him as a reincarnation or a formidable man to create rebellion to overthrow the Abbasid caliphate.
Once in the Sawadh, he and his associates found a ruined Umayyad fort and created a secret HQ there. Al-Tabari said, that al-Dibaj (what we will call him from now on, or Sahib al-Zanj [master of the Zanj]) had traveled around the slaves and felt that if given the opportunity, he could manipulate them into rebellion. The idea, was that he and his inner circle sought rebellion at all costs, they simply lacked an army of foot soldiers. In order to create said army, they would foment a slave rebellion and transfer the slaves into a rebel army with the intent to create a country and overthrow the Abbasid state. To do this, his group split into pairs and move out across the area. They came to slaves and planet the seeds of revolt in the hearts of the overseers, who would bring the slaves to the HQ where they would give submission and they became sleeper cells in their communes, spreading more insurrection among the slaves and sending more groups to the HQ until the word for rebellion was given. Meanwhile, other partisans went to the Bedouin who were inspired by word of a slave revolt ongoing and promised with riches, rewards and the news of a new Imam or prophet who had reincarnated, rebelled against the Abbasid state under the flag of al-Dibaj. Finally, other operatives were sent to Basra, there they fomented civil unrest between the Arab lords in the city, who were turned against each other in petty squabbles. Thus once all of this was completed, al-Dibaj launched the rebellion, attacking estates with his slave army and massacring the lords and their families. In one case, a family of slave holders were chopped to pieces by the slaves by order of al-Dibaj.
2. The Early Phase: This is when the rebellion first begins to pick up steam. Estate by estate was attacked and destroyed by the Zanj rebels who slew the masters and freed the slaves into an ever growing army of warriors which in turn took, according to al-Tabari, Arabs who were not masters as their own slaves (so it was not a rebellion against slavery as a practice). The army swelled and in Basra, the elites attempted to combat the slaves by forming into communal militia. These militia were defeated savagely despite possessing material advantage due to a Zanj ambush.
The victory against the elites, sent the entire coutnryside into rebellion as slaves, Bedouin, bandits, pirates and travelling radicals from abroad joined the rebellion.
3. The Middle Phase: Al-Muwaffaq the caliphal regent and the most powerful general in the state, moved an army into the region to destroy it. He sent a Mamluq slave to protect Basra, while al-Muwaffaq hunted the so-called reincarnated Imam. The war here raged, as the Zanj armies were large by this point and possessed a command structure. They however were pushed aside by al-Muwaffaq and his trained army. This however was stopped when he battled al-Dibaj near al-Mukhantara (the Zanj capitol, which at this time, was just a massive campsite), his army was attempting to be cautious and al-Dibaj, boldly attacked and fearful of being overran, the Abbasid army routed and they were smashed by ambushes attacking them in route, followed by mass deaths from malaria, which destroyed his warriors, especially his Turkic guards unaccustomed to the tropical environment (in this period, Ghilman or Mamluqs were often born and raised in Central Asia or in the European steppe, not raised in the Middle East).
The defeat was followed by a capture of Basra by the Zanj armies. Which was quickly lost due to Zanj religious disputes. The city however was ravaged and did not recover for some many centuries, as according to al-Tabari, the city was systematically deconstructed by the Zanj army and looted beyond imagine. Heads apparently littered the streets and the destruction was incredible and this would highlight the war in general for Arab common opinion latter, an exceptionally brutal war that involved fearsome atrocities on both sides. Once beaten, al-Muwaffaq fled north back to Baghdad and Samarra, where he went to rebuild his army and prepare for the coming Saffarid threat which had rebelled overtly during his defeat in the south. This led to the Zanj becoming the realm ruling all of deep southern Mesopotamia and a power broker that held the keys to conquest of the Abbasid state. Abbasid forces, surrendered their defensive fortifications in the Sawadh and attempted to only contain the revolt, not allowing it to spread north.
4. Zanj ascendancy: Following the rise to power, the Zanj were now free to conclude real conquests. The Zanj forces conquered mcuh of Ahwaz in the east and defeated Ya'qub al-Saffar at the Karun river, pushing back the Persian enemy while asserting its eastern borders. In the south, the Zanj came to rule the entirety of the Nejd, as the Arabs there rebelled and gave fealty to al-Dibaj. Moving west, Zanj forces captured Makkah and Medina and a representative of the Zanj army was placed as lord over the Holy Cities and began making the daily prayers in the name of al-Dibaj and cursing the Abbasid caliph.
Al-Dibaj tuned his campsite into a command center and a large city with many walls and structures. According to al-Tabari, it had well over 100,000 inhabitants by its late stages and dozens of slave markets, selling slaves captured from the Abbasid held lands by Bedouin or by the Zanj armies.
5. The fall:The realm had reached its pinnacle but after the recovery of the Abbasid state and the arrival of al-Mut'amid and his vanguard army, matters made a turn for the worse. He and his father, al-Muwaffaq, were able to fan across the Swadh and over the course of a few years, managed to wear down the zanj state, after fracturing its main army in battle (defeated by a severe cavalry charge by the Abbasid force). Al-Muwaffaq then began to employ a tactic of appeasement, seeking to lure Zanj soldiers to his army with promises of mercy, gifts and food. This succeeded immensely, as the Zanj warriors defected and deserted in large groups, sharing information and positions of their former comrades. This strategy frightened al-Dibaj, who recalled all of his commanders to the capitol which swelled in population and could barely hold itself together.
Al-Muwaffaq and his son then besieged the Zanj city, which would last fro more than a year as the climatic siege moved forward with constant battles occurred in the walls, and a slow push into the city in stages occurred. During the siege, the Abbasid army was constantly enlarging as the Abbasids were joined slowly by their allies from as far away as Morocco and Armenia. Finally, al-Dibaj, trapped, broke through the city and fled, attempting to move south into the Nejd where the rebellion was still strong and unopposed. However, several days earlier, an army of Tulunid warriors had arrived. They were tasked with slaying al-Dibaj who had fled. The Tulunid army thus hunted al-Dibaj and slew him and his army in the field, taking his head to al-Muwaffaq, supposedly ensuring the loyalty of the Tulunids to their Caliphal ovelrords.
---------------------------
Considering this:
If we simply have al-Dibaj slain by a mob in Arabia for blasphemy, we can cut the entire rebellion. After which we can discuss aftermath.
Firstly, Tulunid rebellion is still probable. The Zanj rebellion did more to dissuade the Tulunids than anything else, because it displayed that the Abbasids still possessed the ability to ensure its borders, despite all of its setbacks. At the time, the Abbasid were truly in dire straits. In Anatolia, the Byzantines had slew Umar al-Aqta in 863 and then slew the heads of the Paulician movement, thus annexing much of Arab controlled Anatolia in a tide of victories after slaying the Arab commander. Paulician defeat led to their destruction and alongside the Arab warriors along the front defeated, the Abbasid lost their buffer zone to the Eastern Empire. In the northwest, there was a major revolt in the Jazira region by the Kharijite rebels who had turned the region into a zone of rampant banditry and population movement and trade had come to a standstill. And the Saffarid revolt by the so-called foremost Muhjahid, Ya'qub ibn Layth al-Saffar in Persia. Matters were spiraling out of control in the year 863-880 and the fact that the Abbasid state was able to crush 3/4 of its issues, frightened the Tulunids.
In 876, the Abbasids defeated Ya'qub ibn Layth al-Saffar and then 877-881, they pushed the Zanj into a corner leading to the Tulunids resubmitting themselves to the Abbasids, after having prior rebelled in 868. In 882, the Tulunids had submitted to the Abbasid regent al-Muwaffaq but Ahmad ibn Tulun had yet to surrender his territory in Syria to the Abbasid state. Ahmad ibn Tulun planned to do so, but perished on his journey to Samarra to surrender lands to the Abbasid. His son, Khumarawyah ibn Ahmad al-Tulun assumed the throne at a young age and the Abbasid state felling confident, ended negotiations and sent an army to arrest the Tulunid emir. Kuhmarawyah ibn Ahmad al-Tulun then famously defeated the Abbasid army in the field (despite the young emir feeling to have lost and fleeing on a donkey before the end of the battle [without him there, his army won anyway]).
Without the successes against the Zanj, and others, the Tulunids will not submit again to the Abbasids and will be more less fearful. Instead, they will likely pursue a more aggressive stance. What occurs there is up to chance, both powers are fairly similar, with the Abbasid being stronger overall, but having more interests and thus is unable to focus as easily.
Without the Zanj, the Saffarid campaign may be easier totally different. Saffarids however will be on the offensive, the Abbasids will not move out to attack the Saffarids, as there was a question of the loyalty of Ya'qub al-Saffar still until he explicitly invaded Iraq and challenged the Caliph.
Regarding the topic of the Qarmatians, they may not exist or be nearly as strong. This saves the Abbasid immensely and allows a more powerful Abbasid state entering the next century. The Qarmatians worked much from the Zanj, taking on veterans from their rebellion and also conquering quickly areas left destroyed by the Zanj. Furthermore, the same rebellious instinct in central Arabia which gave the Zanj their power in terms of long term viability, was the same instinct which empowered the Qarmatians to essentially nearly destroy the Abbasid state. I personally, would see the Qarmatian rebellion as a 'part 2' if you will to the Zanj revolt, in some ways, despite being distinct in a myriad of other ways. They are however explicitly related and also convergent in their reliance upon each other.
