The Medinian tribes (largely influenced by conversion of Sa'd ibn Mu'adh) gradually accepted Islam and with Muhammad instituting a spirit of brotherhood, charity, and morality (as opposed to the anarchic, warlike spirit that had, heretofore, prevailed), Islam became a viable growing option for many. As part of survival tactic, the Medinian immigrants were encouraged by Muhammad in their war against Mecca. These, particularly the Battle of Badr, strengthened Muhammad's position and made Islam grow. Heretics were killed and pagans subdued.

Expert strategist and war man, Muhammad prevented alliances from forming against him and devised new military techniques to defend median from siege. Later treaties negotiated, particularly with the Quraysh, further cemented regard for Mohammad, as well as guaranteeing military security for his empire way into the future (Peterson, 2007)

The conquest of Mecca, in 630, resulted in minimal casualties since most of the Meccans...

His famous speech, known as the Farewell sermon, abolished the system of traditional blood feuds and disputes and established the fundamentals of a moral, socially concerned society.
When Mohammed died in 632, he had conquered either by war or by treaty almost all of the tribes of Arabia, many of whom had converted to Islam. During the last years of his life, he had united the tribes of Arabia into a single Arab Muslim religious society led by the Koran and following his example. Single-handedly, Muhammad had brought peace to war-torn Arabia, and he lived on in the Muslim faith as the man closest to perfection and the Prophet of God whom every devout Moslem strives to imitate in all known particulars of his life.

References

Armstrong, K. Islam: A Short History (The Modern Library, 2000)

Hourani, A. A History of the Arab People (London, 1991)

Peterson, D. (2007). Muhammad, Prophet of God. Wm. B. Eerdmans…