From its inception, the Abbasid regime was an unstable collection of Arab and Iranian factions who were unified only by the common impulse to overthrow the Ummayads. Its centralized administrative model ensured that the first century of the Caliphate's existence would be plagued by vicious court intrigues. These intrigues eventually triggered two disasters - the Fourth Fitna and the Anarchy at Samarra. During these disasters, Iranian and Turkic warlords seized the opportunity to carve out de facto independent states, restricting the Caliphate's authority to Iraq.
Part 1: The Rise
The birth of the Abbasid regime started in 747, when the Persian general Abu Muslim al-Khurasani revolted against the Ummayads tnd deefated Caliph al-Marwan's forces in a string of decisive battles. The mission of Abu Muslim's "Black Standard" army was to install the Arab aristocrat Abul Abbas as-Saffah as the New Caliph. Abu Muslim had been in contact with as-Saffah's agents as early as 741, and the two chose the most opportune moment to strike.
By the mid-8th century, the Ummayad government was close to disintegration. The dynasty had always been plagued with dissent due to the stain of the conspiracy which birthed it. In 656, renown admiral and governor Muawiyah revolted against elected Caliph Ali, Muhammad's son in law, on disputable grounds of Ali planning the murder of Muawiyah's uncle, the late Caliph Uthman. Ali, a well meaning but incompetent politician, quickly saw the collapse of his camp, and was forced to name Muawiyah his successor, on the condition that Muawiyah would not let his son Yazid inherit the Caliphate. Muawiyah let his son Yazid inherit the Caliphate. Ali's son Husayn revolted with 72-110 followers, mostly women and children, including a six month old baby. Yazid's troops did not even spare the six month old baby. This spawned the long dispute between Shi'a (supporters of Ali) and Sunnis (supporters of the Ummayads).
Thus, the Caliphate became a hereditary monarchy, and not a very popular one at that. They maintained a policy of Arab supremacy over a mostly non-Arab empire, mandating that any non-Arab who wished to meddle in politics had to become a vassal of an Arab aristocratic family. Additionally, they suppressed local languages in an effort to forcibly Arabize the rest of the Caliphate.
This was resented most of all in Iran, which, unlike the rest of the Caliphate, had been an independent empire for the better part of a millenia, and before that was only briefly occupied by Alexander the Great and his successors. Iranians considered Arabs their intellectual and cultural inferiors, and resented the occupation. Nowhere was this resentment greater than in Khorasan, where Arab settlers and local Iranians were united in a common struggle against the steppe nomads of Central Asia, and thus regularly intermarried, often against Ummayad regulations.
In 747, Caliph al-Marwan had recently seized the Ummayad throne in a devastating civil war, and rebellions broke out around the empire. By 750, the Abbasids had stormed his capital and slaughtered most of his family. The rest of the Ummayad dynasty would be assassinated shortly after, except prince Abd al-Rahman, who escaped to Spain and conquered it to found a rump state.
Part 2: Early Intrigue
The Abbasids' mostly Iranian supporters wasted no time in transforming the Caliphate into an oppressive tax racket centered on its capital. They moved the capital from Damascus to Baghdad, a site only a couple miles from the old Persian capital, and built a palace for as-Saffah in the style of the old Shah's. They insisted he rule through bureaucrats from within the walls of his palace, as the Shahs used to, instead of using the more personal leadership style of earlier Arab Caliphs. The symbolism was not subtle, and the Abbasid dynasty's support in its Western provinces was never strong, as it was always accused of being a resurrected Persian empire.
From the start, the Abbasid Caliphate was plagued by courtly conflicts which gradually degraded the power of the Caliph. The first two Caliphs were powerful due to the shrewdness of al-Mansur, as-Saffah's brother and successor. al-Mansur knew that Abu Muslim, due to his military victories, was now the greatest threat to the regime, and plotted his assassination with the help of an Iranian treasury official named Khalid ibn Barmak. After al-Mansur's death in 775, factional conflicts at court once again rose to the surface.
In 775-76, Khalid ibn Barmak conspired to have al-Mansur's wazir (Prime Minister), al-Rabi' ibn Yunus, replaced with Abu Ubayd Allah. al-Rabi' tried to retake power in 779 by accusing Abu Ubayd's son of being a heretic. He succeeded in deposing Abu Ubayd, but not in regaining the wazirate, as Caliph al-Mahdi appointed his friend Ya'qub ibn Dawud as wazir. Yaqub worked to repair relations with the Arabized Western provinces, but was deposed in 782-83 when Yahya ibn Khalid, son of Khalid ibn Barmak, convinced al-Mahdi that Yaqub was being too soft. In 785, al-Mahdi was murdered by his concubine, probably in a plot by Yahya ibn Khalid. al-Rabi' briefly won the favor of his successor, al-Hadi, before being replaced by Yahya and demoted to chamberlain (secretary).
The Abbasids had a system of brotherly succession since the first and second Caliphs. al-Hadi tried to abolish this pattern by making his son the heir instead of his brother, Harun al-Rashid. This was a problem because Yahya was al-Rashid's tutor and had immense influence over the heir, so he demanded al-Hadi reverse his decision. al-Hadi had the wazir jailed, angering the elite in Baghdad. Rightly fearing for his life, al-Hadi fled Baghdad to Haditha, but was murdered by one of Yahya's agents anyway. Under al-Rashid, the Barmakid family of Yahya ibn Khalid dominated court politics and controlled all important administrative roles. Al-Rabi's son, al-Fadl, inherited his father's position as chamberlain and was the lone voice of dissent.
Part 3: The Fourth Fitna
Tension over the succession would turn court disputes into full-blown civil war. By the turn of the 9th century, tensions between al-Rashid's two sons, al-Amin and al-Mamun had reached a boiling point. Both were tutored by the Barmakids, but al-Amin later came under the influence of al-Fadl ibn al-Rabi'. In 803, al-Fadl convinced Caliph al-Rashid that the Barmakids were getting too powerful, and that they were unduly meddling in the succession in favor of al-Mamun. This led to their sudden purge and fall from power.
The succession crisis could then have resolved in al-Amin's favor had regional tensions not flared up at the same time. The revolving door of wazirs ensured that whoever controlled the wazirate tried to extort as much money from the provinces as possible, before their "time was up". This tax and plunder economy angered all the provinces - none more than Khurasan, which regarded itself as the heart of the Abbasid revolution and was also one of the richest and most urbanized areas of the world at the time. The Barmakid al-Fadl ibn Yahya was sent to calm tensions down in 793, and succeeded due to shrewd governance. However, in 796 he was promoted by his father and replaced by the ravenous Ali ibn Isa. By 806, Khurasan was in full revolt against Ali ibn Isa's taxation. In 808, al-Rashid arrested and replaced Ali ibn Isa, but died en route to Khurasan.
The situation was made worse by al-Rashid's indecisive succession plans, which named al-Amin Caliph in Baghdad and al-Mamun governor of Khurasan and successor. Against the backdrop of a Khurasani rebellion against Baghdad, this could only spell disaster. Al-Mamun quickly enlisted the help of al-Fadl ibn Sahl, a protege of the Barakmids, who convinced him to present himself as the defender of Khurasan and thus induce the rebels to "surrender" to his command. Al-Amin, meanwhile, was popular among the abna al-dawla, the hybrid Arab-Iranian elite in Baghdad. Al-Amin released Ali ibn Isa, who was a prominent member of this group, to solidify his support among them.
In 810, al-Fadl ibn al-Rabi' convinced Al-Amin to replace Al-Mamun as heir with his son. Shortly after, al-Amin sent Ali ibn Isa with 40,000 troops to conquer Khurasan and replace his brother as governor. Surprisingly, Ali ibn Isa was defeated in a string of battles by Tahir the one eyed, a supporter of al-Mamun. Tahir then invaded Iraq, killing the governor of Basra, al-Mullahabi. The desperate ibn al-Rabi' attempted to enlist Arab and Levantine troops, but these men were angered by discrepancies in pay between them and the Caliph's abna' troops, deserting in droves. With his back against the wall, al-Amin armed the civilian population of Baghdad, forcing Tahir the one eyed into a gruelling street to street battle for the city. In 813, Baghdad fell, and Amin was executed, while al-Fadl ibn al-Rabi' went into hiding.
Instead of coming to Baghdad to rule the country himself, al-Mamun stayed in Khorasan and sent ibn Sahl to rule in his name, angering the city immensely. In the West and Arabia, the local Levantine, Egyptian, and Arab populations finally tired of the "Iranian court drama" and backed a number of Shi'a uprisings, attempting to install descendants of Ali. Al-Mamun solved this matter by making an elderly descendant of Ali his heir, calculating that the old man would die first. This, however, enraged Baghdad to no end.
This presented ibn al-Rabi' with the opportunity to make a comeback. He recruited Al-Mamun's uncle Ibrahim to declare himself Caliph with the support of the abna' and emerged from hiding. This revolt was short lived, as Ibrahim's support collapsed when Al-Mamun decided to give the Baghdadis what they wanted and move to the city in 818. In spite of this, ibn al-Rabi' was pardoned. ibn al-Sahl was not so lucky, and was assassinated on al-Mamun's orders the same year for his failure to contain the Ibrahimid uprising.