Islam was founded in the early seventh century, in what is now Saudi Arabia. Over the course of the past 1,400 years it has spread around the world, becoming a truly international religion with more than 1 billion followers. The history of the spread of Islam begins with the conquest of the Middle East centuries ago and continues into the present.

The Birth of Islam

Islam was founded by Muhammad in A.D. 610, when he began to receive revelations from God through the angel Gabriel. Muhammad is revered by Muslims as the last Prophet, who received God's final and correct revelations to humanity. Muhammad was a spiritual and political leader who gained a devoted following in Medina, and then went on to conquer and Islamize Mecca. Shortly after his death in 632, his successors were able to conquer the whole of the Arabian peninsula, and Islam began expanding into neighboring territories.

Early Muslim Conquests

The Quran, Islam's holy book, contains verses that suggest wars of conquest -- jihad -- are condoned by God and are an obligation for Muslims: "Fight those who believe not in Allah nor the Last Day, nor hold that forbidden which hath been forbidden by Allah and His Messenger, nor acknowledge the religion of Truth" (9:29). From 632 until the mid-eighth century, Muhammad's successors -- the Caliphs -- quickly conquered the Arabian peninsula, and then spread farther into the Middle East, conquering Iraq, Syria, Lebanon, Jordan and Palestine. By the early eighth century, North Africa had become part of the Muslim empire; from there, Muslim Berbers invaded and conquered southern Spain in 711. Islam's expansion in the first centuries of the religion was explosive and relentless.

Reasons for the Successful Spread of Islam

One reason for the success of the early Muslim conquests and for the stability of the rapidly growing Islamic empire was that conquered territories were left socially and culturally intact. "People of the Book" especially -- Jews and Christians -- were not forced to convert and maintained a degree of religious autonomy. This cultural leniency, coupled with the political difficulties arising from the incorporation of so many different territories and groups, ultimately led to fragmentation and a weakening of the central power of the empire during the long Abbasid Caliphate of 750-1258.

The Spread of Islam Today

At its largest, the Muslim Caliphate extended from Spain to North Africa to Turkey to Afghanistan. By the early 20th century, the Muslim empire -- the Ottoman Caliphate -- was much smaller, but Islam itself had spread around the globe. Today, as PBS observes, Islam is the fastest-growing religion in the world and will soon be the biggest. While Islam continues to spread itself peacefully throughout the world, aggressive growth and expansion of Islam also continues, especially in parts of Africa -- such as Sudan and Mali -- where militant Islamists battle for territory and political control.