The regions of the world where Islam is the main religion, essentially, northern Africa, the Middle East, South and Central Asia, and Southeast Asia -- principally, Indonesia, Thailand, and the southern Philippine Islands -- represent hundreds of ethnicities, tribes, clans, and individual cultures, and hundreds of millions of people. The one thing they all have in common, however, is their adherence to Islam and their reverence for Allah and the Prophet Muhammad. And that, in a sense, is how the spread of Islam from its birth in the 7th Century through its spread to the farthest reaches of Asia by the start of the 17th Century, came to unify those hundreds of millions of people in a religion presented to the Prophet Muhammad by the angel Gabriel in the holy book known as The Quran.
Perhaps the finest historical example of how Islam became a unifying force was its spread throughout the land of its birth: the Arabian Peninsula, comprised today of Saudi Arabia, Kuwait, Yemen, Oman, Qatar, Bahrain, and the United Arab Emirates. Muhammad's life, subsequent to the revelation of the Quran, was dominated by his efforts, along with his growing army of followers, to spread the Islamic faith as far as possible. While Islam ultimately spread to its farthest reaches through the movement of traders, it was on the Arabian Peninsula where it was most directly imposed by military force, first by Muhammad, and later by the followers of the precursor that would come to be known as the House of Saud, the Arab leader and proponent of Islam Muhammad bin Saud. The House of Saud would suffer intermittant setbacks over the years, but its ultimate success, due in no small part to its alliance with the followers of an 18th Century theologian known as Muhammad ibn Abd al-Wahhab, who advocated a very puritanical interpretation of Islamic scripture.
The spread of Islam throughout the centuries, especially to the Iberian Peninsula, where Muslims, along with Jews, would ultimately run afoul of the bloody religion-inspired period known as the Inquisition, would involve the considerable use of force but, as mentioned, it expansion into Africa and Asia owed a great deal to the opening of trade routes and the increased intermingling of disparate ethnic and religious groups.
While Islam has definitely represented a unifying force throughout the Muslim world, insofar as there are over one billion followers of that monotheistic faith, beyond the basic tenets of Islam -- for example, the giving of alms, the declaration that there is but one God and Muhammad is his Messenger, the requirement during one's life to make the Haj, or pilgramage to Mecca, and so on -- the divisions within followers of Islam are almost as potent as the unifying force the religion provides. Shi'a and Sunni Muslims, for example, are divided by ancient disagreements surrounding the rightful successor to the prophet. Offshoots of different branches of Islam, for example, the Alawites who currently rule Syria, and Sufis, who believe in a mystical form of Islam, are themselves frequent targets of other Muslims who consider them heretics. In fact, one of the bloodiest wars of the 20th Century was the 1980 to 1988 war between Sunni-dominated Iraq and Shi'a-dominated Iran, with the Arab heritage of the Iraqis also competing with the Persian heritage of the Iranians. In short, that which unites Muslims may be no stronger than that which divides them.