Spain, Portugal, and the Papacy had already considered ownership of newfound lands. In 1455, Pope Nicholas V granted the Portuguese exclusive rights to explore lands and islands along the Atlantic Coast of Africa and to claim for themselves everything that they found there. In 1479, in the Treaty of Alcáçovas, Afonso V of Portugal and his son, Prince John, surrendered sovereignty over the Canary Islands to Ferdinand and Isabella of Spain. In exchange, Spain recognized the Portuguese monopoly over African trade and Portuguese sovereignty over the Azores, the Cape Verde Islands, and Madeira. Two years later, Pope Sixtus IV reaffirmed this treaty, specifying that any new discoveries south and east of the Canary Islands would belong to Portugal.

However, John, now John II of Portugal, claimed that the lands discovered by Columbus belonged to Portugal. The Spanish monarchs were having none of it, and they appealed to the new pope, Alexander VI, for the right to colonize and Christianize the areas discovered by Columbus.

Alexander initiated three formal decrees in response. The first, “by the authority of Almighty God,” awarded exclusive  and perpetual possession of the new territories to Spain. The second fixed a north-south line of demarcation running about 350 miles (560 km) west of the Cape Verde Islands. All lands discovered, or to be discovered, west of that line, said Alexander, were Spain’s. By the stroke of a pen, the pope divided continents! His third decree seemed to extend Spanish influence eastward as far as India. This, of course, infuriated King John, whose subjects had only recently succeeded in rounding the tip of Africa, thus extending the Portuguese monopoly into the Indian Ocean.

Fed up with Alexander, * John negotiated directly with Ferdinand and Isabella. “The Spanish monarchs, fearing the ruthless Portuguese and busy enough digesting the New World, were more than happy to seek a reasonable compromise,” says author William Bernstein. So, in 1494, a treaty named after the Spanish town in which it was stipulated was signed in Tordesillas.

The Treaty of Tordesillas maintained the north-south line Alexander had drawn but moved it 920 miles (1,480 km) farther west. Supposedly, all Africa and Asia now “belonged” to Portugal; the New World to Spain. This westward shift of the line, however, brought much of the as-yet-undiscovered land later known as Brazil into Portuguese territory.

The decrees that authorized Spain and Portugal to take possession of and defend newly acquired lands were used as a basis for much bloodshed. These decisions not only ignored the rights of people who lived in those lands—leading to their subjugation and exploitation—but also spawned centuries of conflict among nations over power and the freedom of the seas.