When Muhammad reported his first revelation in Mecca around 610 CE, it was a monotheistic call set against the polytheistic order of a major caravan city, dominated by the powerful Quraysh tribe. Over twelve years the new faith gathered a small body of followers; in the same period the Quraysh responded with economic pressure, social ostracism, and violence.

In September 622 CE, Muhammad and most of his followers migrated to Yathrib, a town some 320 km to the north, later called Medina — "hijra" in Arabic means migration, departure. Yathrib's various tribes and Jewish communities had invited Muhammad to mediate their local disputes. The new Muslim community organised itself there as a *umma*, a coherent body: rules of worship, of law, of defence, and of taxation evolved together.

The Hijra marks the start of Islamic history. Caliph Umar in 638 CE decreed that the Muslim calendar should be counted from this event, and the lunar Hijri calendar remains in use across the Muslim world today. Only eight years after the Hijra, in 630, Muhammad would return to Mecca as a conqueror.