Just three years after Muhammad’s death, in 635 C.E., Islamic armies took over Syria. In 636 C.E. they took over Persia, then Egypt in 640 C.E. Islam spread across northern Africa, to the Red Sea and Indian Ocean from Arabia to East Africa. Islamic forces entered Spain in 711 C.E.
In Spain the Islamic army led by general Tariq was stopped in southern France by the Christian forces of Charles Martel, the grandfather of Charlemaigne. This Battle of Tours in 732 C.E. was a defining one. If the Islamic army had won, Islam would have probably spread across Europe.
Despite losing the battle, Islamic rulers would remain in Spain for almost 800 years. During that time Muslims, Jews and Christians lived in harmony. Islam was also a force in Sicily for about 200 years. That is where the spread of Islam into the West ended.
It did continue to expand in the east, though. It spread to Azerbaijan, Kazakhstan, Afghanistan, Pakistan, northern India, and Bangladesh. It even reached western China.
Part of the reason it spread so easily was because of the ongoing conflicts between the Byzantine Empire, which ruled from Constantinople, and the Persian Empire. These conflicts weakened both Empires. Areas such as northern Africa that were under Byzantine control were far away from the capital, and Byzantine influence was weak. Muslims believe that divine purpose is responsible for its quick and easy spread.
Muslim rule was governed by a caliph, who ruled politically and religiously. Historical periods of Islam rule are separated into caliphates. Each caliphate shift is based on the shift to a different region. Because Muhammad did not leave a male heir, there was much dissension in regards to what should qualify a caliph. These disagreements would be responsible for the split that would mark two distinctly different sects, the Sunnis and the Shiites. Today 10-15% of Muslims are Shiites, and the remaining majority are Sunni.
Part of the success of the spread of Islam was its ability to adapt elements of the new cultures it encountered. It adopted music, architecture and cuisine from the Roman Empire in Syria. It also imitated political structure from the Roman Empire, including development of a refined military.
In 750 C.E. the caliphate shifted to Baghdad, and Muslims adopted Persian music, poetry, architecture, and garden design. These things are very important because they are the channel for artistry in the Muslim world, since images are discouraged or altogether illegal. Translation of Classical texts into Arabic made texts on science and philosophy available. Baghdad became a world center of civilization and taste. This was the golden age of Islam.
Baghdad was invaded and sacked by the Mongols in 1258. The political center of Islam shifted to Egypt. In 1453 Muslims captured the ancient Christian capital of Constantinople, making the center of the Ottoman Empire as well as of the Muslim world until 1921. During that time Islam spread, primarily through trade to southeastern Asia, to what is today Malaysia, southern Thailand, and Indonesia. It also spread to Mindanao, the southern most island in the Philippines.
This expansion made centralized control difficult, and independent caliphates were established. In Spain, the cities of Cordoba and Granada became local political capitals until the Muslims were expelled from Spain in 1492. In India, Delhi became the center of the Muslim Mughal (Mogul) Empire until the British took over. The Ottoman caliphate in Turkey was dissolved in 1924.
So, ultimately it has been the ability of the Muslim culture to shift and move when faced with defeat that has allowed it to thrive and to spread to so many decentralized areas. This is probably also a result of the absolute conviction that Muslims share that Islam is the one true religion. It is their mission to keep the faith alive and fruitful. Without that kind of passion, it would have died long ago.
