From early on, there were three basic tectonic lines that kept Mohammed’s legacy from ever being placid or unified.
(1) Tribal tensions inside Mohammed’s Quraysh tribe, but between different clans. Then tension and aggression between the Quraysh and other Arabs, plus non-Arabs vs. Arabs. It wasn’t racial as such; in those times, “tribal” took care of everything that “racism” means to us today.
(2) Inheritance question: should Mohammed’s family be treated like hereditary royalty, or should the Armies of God be led by whomever could do it well?
(3) Cultural differences between city dwellers and desert nomads, including tax rate unhappiness
By the 3rd Caliph’s reign, there was a revolt in Egypt. The Caliph, Uthman, had displaced Mohammed’s cousin Ali, so already there was a royalist party grumbling in the background. The nomads, some of whose “conversions” were considered suspect by the devout, began to make trouble. They didn’t like the tax burden, so it was easy to blame the non-Sharif Caliph. During a rebellion based in Egypt, which sent an angry delegation to Damascus, Uthman was assassinated.
Ali, Mohammed’s cousin, became the 4th Caliph, and he had two sons to inherit, so it all seemed to be neatened up. It wasn’t, though. There was too much money and power at stake now. Uthman had been from a different clan in the Quraysh tribe, and some of his relations had liked power. His secretary, Marwan, had a son named Muawiya who aspired to be Caliph after Ali.
Muawiya wanted Uthman’s assassins to be put on trial; Ali was afraid it would only create more division. But he was willing to go through a mediation process about the matter, and that by itself was enough to create a division. A hot-headed group of desert tribes “walked away” from the agreement, becoming known as the Kharijites, those who “walked away.” They came back later to assassinate Ali himself.
Muawiya became Caliph, but Ali’s followers and family challenged him. They raised an army and set up his son Hassan as a rival Caliph. The Battle of Karbalah wiped out Ali’s son Hassan as well as the rest of the family. Muawiya re-established his Umayyad clan in power, and their dynasty remained the rulers of Damascus until 750.
However, as we know, Islam was split into a faction that never recognized Umayyad rule. We still have the Shi’ites today; but at that time, they weren’t regionally clustered into Iran. Shi’ite uprisings could happen at any time in any place. During the Middle Ages, there were many factions of Shi’ites who believed that one or another of the descendants of Mohammed’s relatives must be the divinely-inspired Caliph. They were only rarely unified enough to gain political power, but they were always fanatical and tended, often, to be more intensely spiritual than other Muslims.
As to the third fault-line, from the beginning and until the present there has always been a seesaw of power between those who live in cities and the nomads. The first two Caliphs insisted that their followers should stay in tents outside the cities they conquered, but by Uthman’s time, they were permitted to move into Damascus houses. This was one of the original grievances against Uthman, leading to his assassination.
The nomads always prized toughness above everything. They could go long times without food or water, and they could deal with extreme heat and cold. Like all nomads, they owned little: some cooking pots and dishes, their tents, their flocks. Not much else. Their clothing was minimal, as we still see it today: black or white robes and scarves tied onto their heads. A nomad did everything for himself; a city dweller didn’t know how, and had to pay people.
The nomads despised the people of Damascus, Jerusalem, Alexandria and Ctesiphon for owning furniture and huddling next to indoor fires on cold nights. However, after 20 years, their own clansmen were living in these cities. To a nomad, little tax money was needed, but in the cities, they wanted more revenue. Nomads blamed taxes on cities. They also blamed the cities for making people love food, music, wine, physical comfort and safety. All of these things were to blame for high taxes, and they were also sissy.
So it happened over and over, starting with the assassination of Uthman, that some nomad-based group of Muslims would come in and take over. When they did, learning and culture stopped. Vineyards burned. Women got re-veiled and sent home. Slaves were taken, heads rolled.
The Koran was assembled during Uthman’s time, so from then on, depending on whether they owned copies, people could read the stories and sayings of the Prophet. Many of these sayings were generally contradictory, and you could take them as harsh or moderate. The nomads always, but always, took the harshest meaning as correct. Whoever was in power got to decide.