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. But, as we will see in another episode, this trade abruptly declined in the 15th century, and with it, so did Srivijaya. This brings up a key point about Indian Ocean trade, which is that it was indispensable to the creation of certain city-states, like Srivijaya and the city-states of the Swahili coast. Without trade, those places wouldn't have existed, let alone become wealthy and grand.