Islam originated in the city of Mecca, which is on the eastern side of the Arabian peninsula in modern-day Saudi Arabia. The Prophet of Islam, Muhammad, was a native of this city.
As an adult, Muhammad took up the habit of praying alone in the caves near Mecca. One day the angel Gabriel appeared to Muhammad during these prayers and commanded him to recite verses. These holy verses eventually became the Q'uran, or Koran, the holy book of Islam. The monotheistic message of this new revelation shook Muhammad's hometown, for much of the traffic through the town was related to the thriving polytheistic cults centered there. City leaders, dependent on the polytheism for their income, opposed Muhammad's new preaching, especially as be began to gain converts.
After increasing tension, Muhammad moved his followers to Medina, a nearby city, where his message continued to resonate despite the frequent attacks of the Meccans. Eventually, he went back to Mecca, capturing it and clearing out that city's central shrine, the Kaaba, of the shrines to other gods. He then moved to conquer all of Arabia. Though he died soon after his victories in the Arabian peninsula, his religion quickly spread from its birthplace throughout North Africa, the Middle East and Central Asia, becoming a major force in world religion and politics.