Mohammed, assuming he really lived (which I believe but a few renegade scholars challenge), can be placed in the early 600s. He was a merchant living in Mecca, where the dominant religion was pagan. The Kaaba already existed and was probably a shrine for idols. Its presence had already made Mecca a center of trade. Mohammed was a young man married to an older, wealthier, widow. On marrying Khadija, he took over the merchant business her first husband had left.
Around 610, he began to have visions. Khadija believed and encouraged him. Mohammed told people about the visions, but he never wrote them down; he may have been illiterate. (Arabia may have been mostly pre-literate until 600.) The message of his visions was to adopt something much like the Judaism that also flourished on the Arabian peninsula, to embrace charity for the poor and to reject idolatry. Mecca rejected this message and he left in 622.
In Medina, the extended-family tribes were locked in a dispute they could not settle. Like the later medieval Italian cities that only permitted a genetic outsider to be the mayor to prevent feuds, Medina welcomed Mohammed and asked him to settle the dispute. He and his family settled happily in Medina, where there was a strong Jewish presence. Many of Islam’s similarities to Judaism come from this period, when it wasn’t yet clear that he was going to differentiate “his” religion so sharply from theirs.
The turning point came when the pagan population of Medina accepted Mohammed’s new doctrine, but the Jews saw no reason to make any changes. Mohammed, now the leader of a small city, carried out Jewish pogroms. One tribe was massacred, the other exiled with only what they could carry (a big theme for medieval European Jews, too, as well as modern exiled Jews: always being stripped down to the clothes on your back and an overnight bag).
Mohammed used the resources of Medina to lead a war band on Mecca, where he got revenge for their earlier rejection. The Kaaba became a shrine to Allah. (Muslims believe that this had been its original purpose, that the idolatry period had been a corruption.) Then Mohammed led a successful war to unite the Arabian peninsula under Islam; this was accomplished by the time of his death.
Mohammed died in 632; but Khadija had died some time earlier, leaving him free to remarry not just one but eight wives, mostly widows of the Arabian-unification war. But the best-known wife was Aysha, famously married to the old man when she was only a child. Aisha may have been the prettiest, as well as the youngest. She was also the smartest. Still young when Mohammed died, she used her influence to help shape the Koran. Aysha and the supporters who had been with him from the beginning began to assemble Mohammed’s teachings in written form.
The collected sayings of Mohammed reflect several shifts in his thinking, often without attempting to resolve them. Some of the writings are openly admiring of Jews, while others call curses on them. Some writings command them to keep Jewish law, while others carefully shift the Sabbath to one day before or slightly alter the kosher laws. Some of the writings praise women and others curse them.
Some are harsh on adultery, while others—particularly one rooted in a story about Aisha—dictate moderation. The story: Aysha got lost on a long journey once, turning up a day late with a young man who guided her home. She said she got lost while looking for her dropped necklace, so it’s known as the “Affair of the Necklace.” Many of the Muslims accused her of adultery. Mohammed’s previous dictates were for stoning, but he really didn’t want to lose her. At this time, he had a new revelation: four witnesses are required to establish adultery. This makes it nearly impossible to carry out a draconian sentence of death, which moderates an otherwise harsh ruling.
Mohammed had been the civil ruler as well as the religious authority. With all of his wives, he had no surviving sons, only one daughter, Fatima. Some of his closest supporters (like Aysha’s father) had married their daughters to him to increase their influence. Now, they used it. Almost immediately, there was a power struggle between one of Mohammed’s closest friends and his cousin/son in law. It’s complicated, but here’s the basic outline:
The Caliph was the “successor” to the Prophet. Within just a few years, there were competing Caliphs. Over several generations, competitors assassinated Caliphs and their families. The Shi’ite branch of Islam believed that only the genetic kin of Mohammed could lead the faithful, while the Sunn’i majority was willing to follow any strong, orthodox leader. So Islam was fractured at its heart very quickly.
The first four Caliphs were Mohammed’s companion: Abu Bakr (Aysha’s father), Umar, Uthman, and Ali (Mohammed’s son in law and ward). Omar ruled the longest and began the program of conquest, setting out to conquer the two empires that had overshadowed Arabia for centuries.