Nigeria, March 21 -- In 1455, as Portuguese ships crept down the coast of West Africa in pursuit of gold, glory, and dominion during the age of discovery, power was not wielded by the sword alone. Far away in Rome, with ink and seal rather than cannon and sail, Pope Nicholas V issued a decree to King Afonso V of Portugal that would echo across continents and centuries. It was called Romanus Pontifex, a papal bull, the highest expression of authority in the Catholic world, carrying enormous weight in medieval Europe. The bull granted moral sanction to empire, transforming conquest into a divine mandate.

The document granted Portugal sweeping privileges in Africa, including exclusive rights to explore and trade along the West African coast...