They could do this thanks to their well-armed ships, which captured cities by firing cannons into city walls like IRL Angry Birds. But since the Portuguese didn't have enough people or boats to run the Indian Ocean trade, they had to rely on extortion. So Portuguese merchant ships would capture other ships and force them to purchase a permit to trade called a cartaz. And without a cartaz, a merchant couldn't trade in any of the towns that Portugal controlled. For merchants who'd plied the Indian Ocean trade for years in relative freedom, the Portuguese were just glorified pirates, extracting value from trade without adding to its efficiency or volume. So the cartaz strategy sort of worked for a while, but the Portuguese never really took control of Indian Ocean trade. But they were successful enough that their neighbors, Spain, became interested in their own route to the Indies.