It varied depending on the location and period of time. In early Islamic empires, some Muslim rulers were not concerned with conversion to Islam. Conversion to Islam, even in areas under the control of Muslims, was a gradual process fostered through interaction, intermarriage, and missionary efforts emphasizing spirituality (Sufism). There are also areas such as Indonesia (now the largest Muslim-majority country) and parts of Southeast Asia that were never conquered by Muslims but where Islam spread through missionary activity by merchants and Sufis. In many areas currently or formerly ruled by Muslims, large segments of the population have maintained their ancestral religions. For example, Christians have continued to flourish in largely Muslim Lebanon, and Hinduism remained a majority faith through centuries of Muslim rule.
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 violating 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 some Native Americans or African slaves under colonial rule are seen as violations of Christian principles in the eyes of most modern Christians.