Muhammad's followers were initially few in number, and experienced hostility from Meccan polytheists for 13 years. To escape ongoing persecution, he sent some of his followers to Abyssinia in 615, before he and his followers migrated from Mecca to Medina (then known as Yathrib) later in 622. This event, the "Hijra", marks the beginning of the Islamic calendar, also known as the Hijri Calendar. In Medina, Muhammad united the tribes under the Constitution of Medina. In December 629, after eight years of intermittent fighting with Meccan tribes, Muhammad gathered an army of 10,000 Muslim converts and marched on the city of Mecca. The conquest went largely uncontested and Muhammad seized the city with little bloodshed. In 632, a few months after returning from the Farewell Pilgrimage, he fell ill and died. By the time of his death, most of the Arabian Peninsula had converted to Islam.