According to the BBC, early Portuguese explorers were primarily concerned with finding a maritime route around Africa to India and the East. Previously, the country had traded along the overland route through Europe and the Middle East, but the Ottoman conquest of Turkey in the mid-15th century cut off this lucrative trade route.
The first European explorer to round the Cape of Good Hope at the southern tip of Africa was Bartolomeu Dias. Dias' expedition set off in 1487 with three ships and left one behind on the western coast of Africa. Dias made it around the southern tip of the continent, but dwindling supplies and discontent among the crew threatened the expedition. Dias eventually turned around well before entering the Indian Ocean, and he returned to Portugal to report his failure. Nearly a decade later, Dias advised Vasco da Gama, another Portuguese explorer attempting the same feat. This second expedition ultimately reached India in 1498, establishing the maritime trade route and giving Portugal an advantage in the spice trade. Ultimately, the experiences of these two explorers inspired another sailor, Christopher Columbus, to seek royal patronage for an expedition of his own to find a new route to the East.