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Spain and Portugal effectively divided the known world by drawing a north-to-south line of demarcation in the Atlantic Ocean.

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The Treaty of Tordesillas effectively gave Spain and Portugal a free hand to divide and conquer within their sphere of influence. But what were the circumstances that led to this 15th-century document being drawn up in the first place?

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As Portuguese exploration expanded eastwards, one man decided to head west—Christopher Columbus.

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But after learning of Columbus' voyage, the Portuguese king, Dom João II, sent a threatening letter to the Catholic Monarchs reminding them that under the Treaty of Alcáçovas all lands south of the Canary Islands belonged to Portugal, and that included those discovered by Columbus.

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As a proviso, neither power was to occupy any territory already in the hands of a Christian ruler.

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The treaty in this respect was hugely significant in that it allowed for Portuguese exploration and settlement far to the west of the line of demarcation in subsequent centuries and laid a firm basis for Brazil's claims to vast areas of the interior of South America.

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To accommodate them, the Spanish-born pope Alexander VI issued papal bulls setting up this margin from pole to pole 100 leagues (345 mi or 555 km) west of the Cape Verde Islands, off the coast of northwestern Africa and then controlled by Portugal. All lands east of that line were claimed by Portugal. All lands west of that line were claimed by Spain.

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By the late 1400s, much of the west coast of Africa had been claimed for Lisbon by Portuguese navigators, among them Diego Cão, seen here erecting a commemorative padrão, or pillar, along the west coast of Africa.

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But it was Portugal that led the way. Spanish cartographer Gabriel de Valseca's 1439 portolan chart (pictured) documents early Portuguese 'discoveries.' Its depiction of the Atlantic Ocean stretches down to the Rio de Oro (the southern geographic region of Western Sahara) and includes the Atlantic islands of the Azores, Madeira, and the Canaries.

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The Portuguese Empire expanded exponentially throughout the 15th and 16th centuries, originating at the beginning of the Age of Discovery, a period of extensive overseas exploration that also saw Spain seek out new lands beyond the Iberian Peninsula.

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Word of his voyage soon spread throughout Europe, and was certainly not lost on the Portuguese court. Upon his return, Columbus immediately prepared for a second voyage, again sponsored by Isabella I and Ferdinand II.

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Lands mapped south of Cape Bojador included present-day Senegal, Cape Verde, São Tomé and Príncipe, Sierra Leone, and Angola.

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The treaty also recognized Castilian control of the Canary Islands, but confirmed Portuguese possession of the Azores, Madeira, and the Cape Verde islands, and gave the Portuguese rights to any further islands or lands in the Atlantic that might be discovered.

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In 1488, Portuguese mariner and explorer Bartolomeu Dias became the first European navigator to round the southern tip of Africa, where the Atlantic Ocean meets the Indian Ocean.

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Ten years later, in 1498, Vasco da Gama made landfall at Calicut in India. This discovery of a sea route to India was a game-changer for Portugal. It provided access to the lucrative spice routes, which boosted the economy of the burgeoning Portuguese Empire.

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A far less wholesome consequence of Portugal's maritime exploration was the establishment of the European slave trade. Elmina was the first European settlement in West Africa, located in present-day Ghana. The town grew around São Jorge da Mina Castle—the West African headquarters for its trade and exploitation of African wealth built by Diogo de Azambuja in 1482.

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Spain, alarmed by Portugal's accumulation of vast wealth and its domination of the slave trade, had earlier fought Portugal for control of the region and its valuable resources at the naval Battle of Guinea in 1478, during the War of the Castilian Succession.

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The war ended with a Portuguese victory and, with it, recognition by the Catholic Monarchs—Queen Isabella I and King Ferdinand II—of Portuguese sovereignty over most of the West African territories in dispute, acknowledged in the Treaty of Alcáçovas of 1479.

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Columbus, seeking a western sea passage to the East Indies in a bid to capture a slice of the lucrative spice trade, had managed to persuade the Catholic Monarchs to sponsor a voyage across the Atlantic Ocean, which he erroneously believed would take him to Asia.

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In 1494, the Treaty of Tordesillas, named for the city it was drawn up in, was signed between Spain and Portugal. It neatly divided the New World into land, resources, and people, claimed by both nations.

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Rather than risk conflict, both nations agreed to settle the dispute diplomatically. Each sent ambassadors to discuss ways of reaching an agreement aimed at settling the rights of each nation in the Atlantic. What was proposed was radical and quite unprecedented.

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João II advised his Spanish counterparts that he was already preparing to dispatch a fleet across the Atlantic led by Francisco de Almeida (pictured) and take possession of the new lands. Spain, in turn, warned against any attempt to claim the New World for Portugal. The two nations were on the brink of war.

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Dom João II, however, was still not happy with the small print. He believed Portugal's rights in the New World were insufficiently affirmed. More worryingly, perhaps, he felt the Portuguese fleet would not have sufficient room at sea for their African voyages.

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Everybody was recalled to the table and the line redrawn, moved an additional 270 leagues (about 932 mi or 1500 km) farther west in 1506. This enabled Portugal to claim the eastern coast of what is now Brazil.

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In 1500, Pedro Álvares Cabral was the first European to 'discover' Brazil.

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In all, Columbus completed four voyages to the New World. His expeditions inaugurated a new period of exploration, conquest, and colonization that ushered in the era of the conquistadors and Hernán Cortés, the man who caused the fall of the Aztec Empire.

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The conquistadors headed further south in a slow but progressive effort to establish towns and exert cultural dominance over the natives. They stole and plundered: by the late 16th century, gold and silver imports from the Americas provided one-fifth of Spain's total budget. Spanish rule over South America lasted three centuries, only ending in the early 19th century.

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Meanwhile, the Portuguese continued to expand east into Asia. In 1511, Afonso de Albuquerque sailed into Malacca in modern-day Malaysia, the most important eastern point in the trade network. Two years later, the Portuguese flag was raised in China. In 1543, the first contact with Japan was made.

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The legacy of the Treaty of Tordesillas is mixed. On the one hand, it is an important reference not only to Atlantic Ocean history but also to the memory of the world, allowing the meeting of continents and civilizations separated by unknown oceans.

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On the other hand, the document completely ignored the millions of people already living in established communities in the Americas. Indeed, the resulting conquest and colonization proved disastrous for civilizations, such as the Inca, Taino, and Aztec, along with thousands of other communities whose land it was.

Sources: (National Geographic) (Britannica) (UNESCO)

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In June 1494, a document was created that effectively divided the newly 'discovered' lands outside Europe between the Portuguese Empire and the Spanish Empire. Known as the Treaty of Tordesillas, this agreement between the two maritime superpowers would have a profound effect on world history, and prove disastrous for millions. But what exactly did this medieval treaty entail, and who benefitted from it?

Click through and find out more about this remarkable but controversial alliance.

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Instead, of course, Columbus famously landed on an island in the Bahamas in 1492, known by its native inhabitants as Guanahani. He then visited the islands known as Cuba and Hispaniola (the latter being modern-day Haiti/Dominican Republic). Columbus had inadvertently made landfall in the Americas.

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On their side of the line and from the territories of the vanquished Aztecs, the Spanish expanded Spanish rule to northern Central America and parts of what is now the southern and western United States. Other conquistadors took over the Inca Empire. Defeated indigenous people were forced to convert to Christianity.