At Mecca the ummah were a mostly religious community, but after Muhammad led the hijrah to Medina, the ummah became a political community as well. Knowing why and how this happened, as well as the long-term consequences is important to understanding the history of Islam and the Muslims.
The why and how of the ummah becoming a political group is explained in the way the followers of Muhammad were treated in Mecca. Because the Quraysh were pagans in the majority, they would never let Muhammad or his followers gain power as long as they were the minority. On page 173 Hodgson states, "...the pagan Quraysh could not have tolerated a movement which attacked the principles of their social order [...] So long as the Muslims stayed as a minority in Mecca there would have been a deadlock at best." Their new life needed to "be lived by a society at large," (Hodgson, 173). So when Muhammad and his Meccan followers left for Medina they formed their own social groups, called Muhajirun.
After arriving in Medina Muhammad first set about expelling the Jewish clans who lived there--starting with the Banu Qaynuqa and moving on to the others until they all agreed to leave Medina, (Hodgson 177). When the ummah of both Mecca and Medina became the ruling power (with Muhammad at their head of course) they established clans and superstructures as well as laws and ways of living that helped each other out. For example, zakat--an alms tax--was collected and given to those Muslims who were financially weak, (Hodgson, 181).
Still though, a universal and democratic law needed to be developed--the Qur'an could not answer every problem and internal squabble. Muhammad personally made the law that is now known as the "Constitution of Medina." Hodgson, on page 183 says this, "The 'constitution of Medina,' which settled...