Muhammad oversaw the conquest of the cities of Mecca and Medina, which in turn dominated the Hejaz region along the eastern shore of the Red Sea. His charisma kept the followers united, but after his death, many of the Arab tribes he had led turned out to be less than devoted. Without a definite winner, the tribes reverted to their autonomous raiding methods. In order to bring the many tribes back together under one political flag, Abu-Bakr, who succeeded Muhammad as caliph, waged war. Abu-Bakr was aware that since Islam outlawed combat amongst believers, the Bedouin ways of robbing and pillaging needed to be changed. They must therefore locate unbelievers to attack.
In Palestine, Abu-Bakr contested the Byzantine Empire's dominance. On July 30, 634, between Jerusalem and Gaza, Muhammad dispatched his strongest commander, Khalid, on raiding parties that eventually came together to destroy a bigger Byzantine force.
Umar (Omar, 634-644), Abu-successor, Bakr's seized Jerusalem. He therefore dispatched troops in all directions to oppose the might of both Constantinople and the Sassanid Empire in Persia. Once more, Khalid was victorious, seizing Damascus by deceit in 635 and settling in Emesa (modern-day Horns) by the end of the year. The following spring, he surrendered the city to a 50,000-man Byzantine force, but in August 636, he outmanoeuvred and destroyed them. Despite being twice as large as Khalid's forces, the Byzantine army faced a hostile populace that had grown enraged over years of taxes and religious oppression. Despite not being Muslims, the populace embraced the invaders as liberators from Constantinople's oppression.
With the exception of the occasional autonomous activities like the invasion into India, the Muslims consolidated their hold for a few centuries as opposed to expanding it. They benefitted in all of their Middle East campaigns from the military adequacy of their adversaries as well as the assistance of resentful subjects. Although this only sometimes occurred, the Muslims had a reputation as conquerors who forced their faith upon the downtrodden. Muslim sultans generally adhered to Muhammad's instructions to respect the rights of people of other religions. However, the imposition of a tax on nonbelievers led a large number of the impoverished to convert, starting the long history of the Islamic faith in that region.
References:
Armstrong, Karen, Holy War (New York:
Macmillan, 1988); Koprulu, Mehmed Fuad, Islam
in Turkey after the Turkish Invasion (Salt Lake City:
University of Utah Press, 1993); Serjeant, R. B.,
Studies in Arabian History and Civilisation
(London: Variorum Reprints, 1981).