Quraysh
Muhammad's own tribe, who ran Mecca, fought him for two decades, then converted and supplied nearly every caliph for six centuries.
The Quraysh were the tribe that controlled Mecca, its shrine, and its trade in the century before Islam. Muhammad was born into them. So were his fiercest enemies, his first followers, his successors, and the two dynasties that ruled the Muslim world for the next six hundred years. Almost every major conflict in early Islamic history is a fight between branches of one extended family, and knowing which branch is which makes those conflicts legible instead of confusing.
Custodians of the shrine
By tradition the Quraysh were settled in Mecca in the fifth century by an ancestor named Qusayy, who took control of the Kaaba, the ancient shrine that drew pilgrims from across Arabia. That gave the tribe two assets. The first was religious prestige: they were the guardians of a sanctuary in which the surrounding tribes housed their own gods and inside which violence was forbidden. The second was commercial: pilgrimage traffic and the sacred truce made Mecca a natural market, and Quraysh merchants ran caravans north to Syria and south to Yemen.
The tribe divided into clans, and two matter more than the rest. The Banu Hashim, Muhammad's clan, were respected but not rich. The Banu Umayya were the wealthiest and most powerful. That imbalance runs through everything that follows.
Enemy, then establishment
When Muhammad began preaching one God in around 610, the Quraysh leadership opposed him for reasons that were as much economic as theological. A prophet who declared the idols in the Kaaba to be nothing threatened the shrine's pull, the pilgrim traffic, and the tribal order that gave Mecca's leading families their authority. They boycotted his clan, harassed his followers, and finally drove him to Medina in 622.
What followed was eight years of war between Muhammad's community and his own tribe: Badr in 624, Uhud in 625, the siege of Medina in 627. The Quraysh commander for much of it was Abu Sufyan of the Banu Umayya. When Muhammad took Mecca in 630, he did not take revenge. He declared a general amnesty, and the Quraysh elite converted and were absorbed into the new order at senior rank. Abu Sufyan's son Muawiya became a scribe to the Prophet, then governor of Syria, then the first caliph of the Umayyad dynasty.
That amnesty was politically brilliant and permanently destabilizing. Within thirty years, the men who had fought Islam longest were running it. The early Companions who had suffered in Mecca watched Muhammad's former enemies take the richest governorships, and the resulting fury produced the murder of the caliph Uthman, himself an Umayyad, and the first civil war.
The bloodline as constitution
The Quraysh connection then became formal doctrine. Sunni jurists held that the caliph must be of Quraysh, citing a report attributed to the Prophet to that effect. Shia Islam narrowed the requirement further, restricting legitimate authority to the descendants of Ali and Fatima, which is to say the Banu Hashim.
So the great dynasties advertised their descent. The Umayyads ruled from Damascus as Quraysh of the Banu Umayya. The Abbasids who overthrew them in 750 were Quraysh of the Banu Hashim, descended from the Prophet's uncle, and made that claim the core of their legitimacy. When the Ottoman sultans, who were Turks with no Qurayshi ancestry, claimed the caliphate, they had a doctrinal problem their scholars had to argue around for centuries.
The claim still pays. The Hashemite kings of Jordan take their name and much of their standing from descent from the Prophet's clan, as did the Hashemite kings of Iraq before 1958. The Moroccan monarchy makes the same claim. When the Islamic State proclaimed a caliphate in 2014, it gave its leader the name al-Qurashi and asserted Qurayshi descent, because the classical requirement had never been repealed. A tribal genealogy from pre-Islamic Mecca is still doing constitutional work in the twenty-first century.