There are several theories to the Islamisation process in Southeast Asia. Another theory is trade. The expansion of trade among West Asia, India and Southeast Asia helped the spread of the religion as Muslim traders from Southern Yemen (Hadramout) brought Islam to the region with their large volume of trade. Many settled in Indonesia, Singapore, and Malaysia. This is evident in the Arab-Indonesian, Arab-Singaporean, and Arab-Malay populations who were at one time very prominent in each of their countries. The second theory is the role of missionaries or Sufis.[citation needed] The Sufi missionaries played a significant role in spreading the faith by introducing Islamic ideas to the region. Finally, the ruling classes embraced Islam and that further aided the permeation of the religion throughout the region. The ruler of the region's most important port, Malacca Sultanate, embraced Islam in the 15th century, heralding a period of accelerated conversion of Islam throughout the region as Islam provided a positive force among the ruling and trading classes.