This process varied depending on the location and historical period. Islam in its early years unified the tribes of the Arabian Peninsula, and this new unity led to conflict with the nearest major powers, the Byzantine and Persian empires. The result was a major spread of Muslim rule and the establishment of a Muslim empire; but Muslim rulers in this empire did not force, and often did not even encourage, conversion to Islam.
Conversion to Islam, even in areas under the control of Muslims, was a gradual process that took place over many centuries and was fostered through interaction, intermarriage, trade, and efforts by Sufis (spiritual seekers). Professor Ira Lapidus in his book, A History of Islamic Societies writes: “The question of why people convert to Islam has always generated intense feeling. Earlier generations of European scholars believed that conversions to Islam were made at the point of the sword and that conquered peoples were given the choice of conversion or death. It is now apparent that conversion by force, while not unknown in Muslim countries, was, in fact, rare. Muslim conquerors ordinarily wished to dominate rather than convert, and most conversions to Islam were voluntary.” (p. 198)
In areas like Indonesia (now the largest Muslim-majority country) and other parts of Southeast Asia, Islam spread mostly through traveling merchants and Sufis. In sub-Saharan Africa (mostly West Africa, but also parts of Ethiopia), Islam spread mostly through trade and commercial relations. Rulers would sometimes adopt Islam while much of the population continued to practice their traditional religions. In many areas currently or formerly ruled by Muslims, large segments of the population have maintained their ancestral religions. For example, Christians are a significant minority in largely Muslim Lebanon, and Hinduism remained a majority faith through centuries of Muslim rule in South Asia.
This is not to say that Muslims have never violated the principle stated in the Qur’an that “there is no compulsion in religion.” Some forced conversions occurred, for example, in the Horn of Africa during the 17th-century wars between Christian Ethiopians and Muslim Somalis, as they did in other times and places.
Today we believe that forced conversions or denying the religious rights of people of other faiths are as much a violation of Islamic principles as the forced conversion of the Germanic tribes under Charlemagne or the forced conversions of Native Americans or enslaved Africans are seen as violations of Christian principles in the eyes of most modern Christians.