Final answer:
Portugal held supreme maritime power in the 15th century due to its explorations and control over the new sea routes around Africa and Asia. Spain's influence grew with Columbus's voyages and Magellan's circumnavigation, beginning their rise to maritime dominance in the 16th century.
Step-by-step explanation:
The country that held supreme maritime power in the 15th century was Portugal. This was a time characterized by significant explorations and the beginning of global ocean trade. The Portuguese were pioneers in maritime exploration, starting by claiming islands off the coast of Africa and venturing down its western coast. The Portuguese explorer Bartolomeu Dias was the first to sail around the southern tip of Africa in 1488, paving the way for further exploration towards India and the eventual establishment of a sea route to the east. Portugal's control over this maritime route, marked by the Treaty of Tordesillas and its consequent exclusivity to trade with the eastern territories, notably bolstered its position as the dominant maritime power of the late 15th century.
However, it was during the reign of Spain in the 16th century that the Spanish Armada would rise and Spain would begin to establish itself as a formidable maritime force, especially after the expeditions of Christopher Columbus, which were underwritten by King Ferdinand and Queen Isabella of Spain. The successful circumnavigation of the globe by Ferdinand Magellan, in the service of Spain, further supported Spain's claim to global maritime dominance in the following century.