Knowing what you've already learned about the growth of Islam and the spread of trade, it won't surprise you to learn that Islam spread to Indonesia via the monsoon marketplace. After the 1200s, the region, which had previously been heavily influenced by the Indian religions of Hinduism and Buddhism, like witness this temple, for instance, became increasingly Islamic as rulers and elites adopted the religion so they could have religious as well as economic ties to the people they were trading with. The conversion of most of a region to Islam, where it continues to flourish today, is a pretty big deal. But Islam didn't spread as effectively to Thailand, Laos, Cambodia, or Vietnam because they weren't centers of trade. How do you become a center of trade? Well, let's zoom in here, to the Strait of Malacca. You can see how it could act as a choke point for trade.