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. Any city that controlled that strait could stop ships from going through it, or at least tax them. And that's exactly what happened, to such an extent that a powerful merchant state called Srivijaya rose up on Sumatra. And for a while, Srivijaya dominated trade in the region because there were so many ships going through the Strait of Malacca to and from China.