Perched on the southwestern part of the Iberian peninsula, Portugal turned to the boundless Atlantic Ocean as its only outlet to the wider world. As early as 1341, Portuguese sailors had made their first forays into the tempting waters that lay beyond their shores, exploring the Canary Islands off the northwestern coast of Africa.

Rival Spain would later end up conquering the Canaries, but the Portuguese had already seized the global advantage when it came to shipbuilding, navigation and mapmaking. Not long after the 15th century dawned, Portugal under the ambitious King John I turned its sights toward Morocco, the Muslim stronghold seen as the gateway to the gold, spices and other untold riches in Africa and beyond.

In 1415, a Portuguese fleet crossed the Strait of Gibraltar and captured the heavily fortified Moroccan port of Ceuta, announcing Portugal’s arrival on the world stage. In the decades to come, John’s son Prince Henry the Navigator financed numerous expeditions along the western coast of Africa, aimed at spreading Christianity and making Portugal rich with profits from gold, spices and slaves. Portugal was largely responsible for introducing the slave trade to the Americas through colonies on previously uninhabited Atlantic African islands that served as collection points for captives and commodities.

By the time Henry died in 1460, Portuguese sailors and settlers had reached as far as modern-day Sierra Leone, and formed active colonies on the islands of Porto Santo, Madeira and the Azores.

Momentum behind Portuguese maritime exploration slowed somewhat after Henry’s death, but would regain strength under the rule of his grandnephew, King John II. In 1487, on a mission to find a water route from Portugal to India, Bartolomeu Dias led the first successful sea voyage to the southern tip of Africa, rounding the Cape of Good Hope and sailing for a few days before turning back.

Back in Lisbon in late 1488, the Genoese navigator Christopher Columbus was among those in the audience as Dias shared tales of his historic voyage with John’s court. Columbus, who received his training in navigation in Lisbon and had been married to a Portuguese woman, tried to interest John in his own proposal to find the resource-rich Indies by sailing west. But the Portuguese king rejected this idea, leaving Columbus to seek support from John’s rival monarchs, Isabella and Ferdinand of Spain.

In 1494, the year after Columbus made his triumphant return to Europe after reaching what are now the Bahamas, Cuba, Haiti and the Dominican Republic, negotiators from Portugal and Spain met in a small Spanish town to divide up control of what they called the “New World.”

According to the Treaty of Tordesillas, a vertical line was drawn through the Atlantic Ocean about 345 miles west of the Cape Verde Islands, located off the northwestern African coast and controlled at the time by Portugal. Spain claimed all lands to the west of the line; Portugal all lands to the east, including the coast of Brazil, which at the time had not yet been officially “discovered.” (Portuguese explorer Pedro Álvares Cabral would reach Brazil in 1500, prompting speculation by historians that in fact Portugal already knew of its existence from an earlier expedition, and had used that knowledge to push the treaty’s boundaries further west.)

Though Spain and Portugal largely respected the Treaty of Tordesillas, it would be ignored by other European powers—including Britain, France and the Netherlands—going forward. In addition, the treaty completely disregarded as many as 50 million people who were already living in the Americas, and who would suffer the devastating consequences of European expansion.

In 1497, Vasco da Gama led four ships and nearly 170 crew members along the route Dias had followed, this time veering even more sharply into the southern Atlantic to catch the favorable currents needed to get past the Cape of Good Hope. Plagued by hunger, scurvy and other perils of the journey, they sailed up the eastern coast of Africa, stopping at Mozambique and other ports in modern-day Kenya. With the help of a local navigator, da Gama and his ships made it across the Indian Ocean, reaching Calicut, India, in May 1498.

Da Gama’s success opened the first water route to India from Europe, paving the way for a new era of global trade and colonialism. On later expeditions, da Gama and others established a Portuguese network of trading posts and fortresses in eastern Africa and India, using brutal force against local Muslim and Hindu populations when they saw fit. Lisbon’s harbor soon bustled with ships carrying prized spices like cinnamon, ginger, black pepper and saffron, along with other precious goods.

In the early 16th century, Portugal was the most prosperous nation in the world, thanks to its feats of navigation, exploration and conquest. From India, its ships pushed further east, reaching the Spice Islands (Indonesia) in 1512 and China in 1514.

A few years later, the sailor and navigator Fernão de Magalhães (anglicized as "Magellan") proposed taking a westward route to the Spice Islands around the tip of South America. After Portugal’s King Manuel I rejected him, Magellan (like Columbus before him) turned to Spain instead.

Magellan died in the Philippines, but one of his ships made it back to Spain in 1522, completing the historic effort to circumnavigate the globe and marking the beginning of the end of Portugal’s dominance of the seas.