While looking for a sea passage to India, it would be helpful to have a strong Christian ally in another region. The land expedition eventually did find a Christian kingdom -- not Prester John's but that of Alexander, King of Ethiopia.
In 1488, the sea expedition, under the command of Bartolomeu Dias, sailed down the African coast and reached Cape Town. This opened the way for Vasco da Gama's voyage to India, which was also ordered by King John.
These successful expeditions set the stage for sea travel and trading between Europe and Asia.
Christopher Columbus made many efforts to persuade King John II to support his own expedition west across the Atlantic Ocean. The King, who had given the project to a commission of "experts," turned him down several times, notably after Dias's triumph. Columbus then left for Spain, where he finally found support from Ferdinand and Isabella.
