In the summer of 621 CE, 12 men from Medina, visiting Mecca for the annual pilgrimage to the Ka'bah (still a pagan shrine), secretly professed themselves Muslims to Muhammad and went back to make propaganda for him at Medina.

At the pilgrimage in June 622 CE a representative party of 75 persons from Medina, including two women, not merely professed Islam but also took an oath to defend Muhammad as they would their own kin.

These two women are known as the Two Pledges of al-'Aqaba.

Muhammad now encouraged his faithful Meccan followers to make their way to Medina in small groups, and about 70 emigrated thus.

The Meccans are said to have plotted to kill Muhammad before he could leave.

With his chief lieutenant he slipped away unperceived, used unfrequented paths, and reached Medina safely on September 24, 622 CE.

This is the celebrated hijrah (Latin Hegira), which may be rendered "emigration," though the basic meaning is the severing of kinship ties.

It is the traditional starting point of Islamic history.

Medina was vastly different from Mecca.

It was an oasis in which date palms flourished and cereals could be grown.

Agriculture had been developed by several Jewish clans, who had settled among the original Arabs, and they still had the best lands.

Later Arab immigrants belonging to the tribes of al-Aws and al-Khazraj, however, were in a stronger position.

The effective units among the Arabs were eight or more clans, but nearly all of these had become involved in serious feuds.

Much blood had been shed in a battle in about 618 CE.

In inviting Muhammad to Medina, many of the Arabs there probably hoped that he would act as an arbiter among the opposing parties.

Their contact with the Jews may have prepared them for a messianic religious leader, who would deliver them from oppression and establish a kingdom in which justice prevailed.

A document has been preserved known as The Constitution of Medina. In its present form it is a combination of at least two earlier documents and is probably later than 627 CE, but its main provisions are almost certainly those originally agreed upon between Muhammad and the Muslims of Medina.

In form the document creates a confederation on traditional Arab lines among nine groups--eight Arab clans and the emigrants from Mecca.

Muhammad is given no special position of authority, except that the preamble speaks of the agreement as made between "Muhammad the prophet" and the Muslims now resident in Medina, and it is stated that serious disputes are to be referred to him.

For at least five years, Muhammad had no direct authority over members of other clans, but, in the closing years of his life, the prestige of his military successes gave him almost autocratic power.

The revelations he received at Medina frequently contained legal rules for the community of Muslims, but they dealt with political questions only rarely.

The Hijrah or Hegira (flight)  to Medina

Hijrah is also spelled HEJIRA, or in Arabic HIJRAH, or HIJRA ("flight," or "emigration").

The Hegira or Hijrah represents the Prophet Muhammad's migration (CE 622) from Mecca to Medina in order to escape persecution.

This date represents the starting point of the Muslim era or calendar.

Muhammad himself dated his correspondence, treaties, and proclamations after other events of his life.

It was 'Umar I, the second caliph, who in the year 639 CE (AH 17) introduced the Hegira era (now distinguished by the initials AH, for Latin Anno Hegirae, "in the year of the Hegira").

'Umar started the first year Ah with the first day of the lunar month of Muharram, which corresponded to July 16, 622 CE.

The term hegira has also been applied to the emigrations of the faithful to Ethiopia and of Muhammad's followers to Medina before the capture of Mecca.

Muslims who later deserted lands under Christian rule were also called muhajirun ("emigrants"). The Khawarij (Kharijites), those Muslims who withdrew their support from the arbitration talks that called into question the right of the fourth caliph, 'Ali, to the caliphate in 657CE, also used the term to describe those who joined them.

Those who emigrated to Medina with Muhammad  are considered most honored muhajirun.  The  and are known as the Companions of the Prophet.  Muhammad praised them highly for having forsaken their native city and followed him and promised that God would favor them.

They remained a separate and greatly esteemed group in the Muslim community, both in Mecca and in Medina, and assumed leadership of the Muslim state, through the caliphate, after Muhammad's death.

As a result of the Hegira, another distinct body of Muslims came into being, the ansar ("helpers"). These were Medinese who aided Muhammad and the muhajirun.

The ansar were members of the two major Medinese tribes, the feuding al-Khazraj and al-Aws, whom Muhammad had been asked to reconcile when he was still a rising figure in Mecca.

They came to be his devoted supporters, constituting three-fourths of the Muslim army at the Battle of Badr (624 CE).

When no one of their number was chosen to the caliphate to succeed Muhammad, they declined in influence as a group and eventually merged with other Muslims who had settled in Medina.