Islam spread quickly beginning in the seventh century through two primary means. The first method was conquest. Although no one was forced to convert after Muslim conquests, many found it advantageous, and some of the areas were eventually Islamized. The second method was trade, which spread the message of Islam through merchants.
Conquest: Muhammad and Arabia
The first Islamic community was founded at Medina in 622, an event which begins the Muslim calendar. From Medina the Muslims branched out, eventually capturing Mecca in 630. By the time of his death in 632, Muhammad had united the entire Arabian Peninsula under his new religion. Abu Bakr, Muhammad's first political successor, or caliph, put down rebellions in Arabia and set the stage for the rapid expansion that would follow.
Conquest: The Expanding Caliphate
The caliphate expanded rapidly under Umar's reign from 634 to 644, spreading north to Mesopotamia, east to the Persian lands of the defeated Sassanid Empire and west across North Africa. By 750, under the Umayyad caliphate, the Muslim world had spread through North Africa up to southern France, north to the Caucasus and east into modern-day Afghanistan and Pakistan. Spain was eventually reconquered by Christian forces, but for the most part, all of the land conquered in this era remains Muslim today. The Ottoman Empire would eventually supplant the Byzantines in modern-day Turkey, overseeing an empire that once stretched from the Balkans to Somalia, and from modern-day Morocco to Persia.
Trade: Sub-Saharan Africa
Islam in East Africa dates back to the early seventh century, when Muslims took refuge from Meccan persecution in modern-day Ethiopia. North African traders brought Islam to West Africa as early as the eighth century, fueling a gradual Islamization of the area. Why so many West Africans eventually converted to Islam is unknown, but three theories are generally given: economic motives, the appeal of Islam's spiritual message and the prestige of Arabic society. Islam was similarly spread down the East Coast of Africa as Muslim traders intermarried with the local population and the area's elite converted to Islam. Islam became a sign of social status and economic opportunity on the East African coast.
Trade: Malaysia and Indonesia
Indonesia is the world's largest Muslim-majority country and the last major region to be converted to Islam. Muslim trade ships brought Islam in the 14th century, and soon many Sufis – Islamic mystics – began to move inland to preach the faith. The faith spread in a similar manner in modern-day Malaysia. Later European colonization stopped the spread of Islam in the rest of Southeast Asia, but the roots of Islam were deep enough to survive the colonization of the Dutch and the British.
To All Corners
Trade also brought Islam to places that never had a majority Muslim population. Islam was spread via the Silk Road as far as the city of Chang-an in eastern China. Trade also brought Islam to India, which has one of the world's largest Muslim populations despite its Hindu majority. In modern times, immigration and economic opportunity have brought Muslim communities to traditionally Christian areas such as Europe and North America.