Why Did King John of Portugal Believe That the Newly Found Lands Should Belong to Portugal Instead of Spain? The Treaties, Bulls, and Atlantic Logic Behind a 1490s Superpower Dispute

King John II of Portugal believed the newly found lands belonged to Portugal because Portugal had received papal grants to conquer and evangelize lands south and west of a line near the Canaries, because Portuguese mariners had explored the Atlantic first, and because the 1479 Treaty of Alcáçovas had already divided the Atlantic into Spanish and Portuguese spheres. When Columbus returned in 1493 claiming lands for Spain, John argued those islands fell inside Portugal's zone — a claim rooted in treaty text, papal bulls, and prior discovery, not mere royal ambition.

The dispute that followed reshaped the map of the world. It produced the Bull of Demarcation, the Treaty of Tordesillas, and a diplomatic precedent that lasted for centuries. Understanding why John believed what he believed means understanding three interlocking systems: papal patronage, bilateral treaty law, and the practical geography of Atlantic sailing.

The mechanism: papal grants created property-like claims

Medieval and Renaissance Europe treated overseas expansion as a legal act, not just a voyage. The pope, as spiritual head of Christendom, could grant monarchs the right to conquer, colonize, and Christianize non-Christian lands. These grants were called bulls, and they functioned as a kind of international title deed.

Portugal had been accumulating such grants since the 1430s and 1440s. Pope Eugene IV and Pope Nicholas V issued bulls confirming Portugal's rights to lands discovered or to be discovered along the African coast and in the Atlantic. The most relevant was Romanus Pontifex (1455), which granted Portugal extensive rights to trade and conquer in the regions "from the Capes of Bojador and Nam and through all Guinea, and past that southern limit to the Indies." In practice, this established a Portuguese monopoly on Atlantic and African exploration under papal authority.

This is the crucial mechanism: papal bulls did not describe fixed coordinates. They described directions and zones. Portugal's grants were framed as "south and west of the Canary Islands," which was geographically vague but legally powerful. When Columbus returned claiming Caribbean islands, King John could argue those islands were south and west of the Canaries — and therefore inside Portugal's papal zone.

The bull system worked because there was no other agreed international law for overseas territory. The pope was the only authority with universal jurisdiction in Latin Christendom. A papal grant was enforceable because violating it meant defying the Church. Strengthened by the Treaty of Alcáçovas, Portugal's legal position was, in its own view, airtight.

Quantified context: dates, distances, and the treaties that mattered

The chronology matters because each step built on the last. Portugal's claims were not improvised in 1493; they were the product of decades of accumulation.

The 1479 Treaty of Alcáçovas was the foundation. It ended the War of Castilian Succession and, among other terms, recognized Portuguese possession of the Azores, Madeira, Cape Verde, and Guinea. Critically, it assigned Castile the Canary Islands — which lie roughly 100 miles off Morocco — while reserving for Portugal all lands "discovered or to be discovered" south of the Canaries and west toward Guinea.

That phrase is the key. It is a forward-looking clause. It didn't just settle present territories; it allocated future ones. To King John and his jurists, Columbus's islands, lying in the Caribbean and described by Columbus himself as fitting a southward and westward route, fell squarely inside the Portuguese side.

The distances involved were large: the Caribbean lies some 3,000 nautical miles from the Iberian peninsula, while the Azores sit about 850 miles west of Lisbon. The bulls operated in leagues — a league in this era was roughly 3 nautical miles, though regional variation made precise measurement unreliable. That imprecision would later become a problem.

How John II argued his case: discovery, titles, and prestige

Portugal's case rested on more than legal text. It rested on a doctrine of prior discovery and prior papal recognition, reinforced by national prestige.

Portugal had been the leading Atlantic exploring power for roughly 70 years. Under Henry the Navigator and his successors, Portuguese ships had pushed down the African coast, reached the Cape Verde islands by the 1460s, and were approaching the Cape of Good Hope by the late 1480s. Portuguese sailors had mapped currents, winds, and coastlines. King John had himself sent explorers west — the 1486 expedition that may have sighted land in the western Atlantic.

This mattered because fifteenth-century legal thinking treated discovery and first possession as real grounds for title. In the absence of a world court, who got there first and who could demonstrate sustained use mattered. John could argue that Portugal had pioneered Atlantic routes, held papal approval for Atlantic conquest, and therefore had a prior legal claim on any new islands in the region.

The counterintuitive detail here is that King John did not initially claim the mainland of a new continent. Columbus's accounts described Caribbean islands, which fit the Portuguese claim more easily than a continental landmass. John argued about the islands, and specifically about their location relative to the Canaries. He was prepared to negotiate, but from a position of legal strength.

Edge cases: where John's argument was strong and where it strained

John's case was not uniformly strong. It had genuine legal footing but also genuine weaknesses that forced him to negotiate.

Strong points:

The Treaty of Alcáçovas explicitly reserved lands south of the Canaries for Portugal. Papal bulls framed Portugal's claim in terms of direction, not coordinates. Portugal had decades of prior exploration and settlement in the Atlantic. Spain had no prior papal grant covering the Caribbean at the time Columbus returned.

Weak points:

Alexander VI, the pope who issued the 1493 bulls, was Spanish (Rodrigo Borgia), and his Inter caetera leaned toward Spain. Columbus claimed to have sailed west, not south — a geographic argument that could place the islands outside Portugal's zone. The precise location of the islands was unknown and disputed, and fifteenth-century navigation could not fix coordinates with confidence. No existing treaty specified where Portugal's southern zone began along Earth's curving surface.

These weaknesses explain why John chose negotiation. Rather than wage war over the Canaries or the Caribbean, he agreed to diplomatic talks. The result was the Treaty of Tordesillas, which moved the demarcation line about 270 leagues further west than the 1493 bull line — a meaningful concession to Spain in many interpretations, but possibly a calculated one for Portugal.

Red flags worth noting for anyone reading these treaties:

Medieval and Renaissance treaties used direction words ("south," "west") as proxies for coordinates, and these directions are ambiguous at a global scale. Leagues were not standardized, so "370 leagues" could mean different distances depending on regional usage. Papal grants could be revoked or reinterpreted by successive popes, making them unstable legal foundations. Maps were partial and sometimes inaccurate, so a line drawn on a 1494 chart did not map cleanly onto the real world. Treaties settled disputes between states, but not with other emerging powers, so their authority was limited in practice.

Misconceptions: fortune-telling and digital legends

Several popular ideas misunderstand why King John believed what he believed. The one about a foresighted negotiating strategy is among the most durable.

One common misconception is that John foresaw the value of Atlantic lands and negotiated expertly to secure a future empire. There is something to this in retrospect, but it is partly hindsight. John's primary concern in 1493–1494 was defending existing Portuguese claims and defending Portugal's reputation as a world power. Treating the treaty as a grand strategic design overstates the evidence.

Another misconception is that the Treaty of Tordesillas was a clean division of the world into two equal halves. It was not. It was a line in the Atlantic whose east-west extent was debated for decades. Its eastward interpretation gave Portugal a claim — eventually realized — to Brazil, but that outcome was a consequence, not the original intent.

A third misconception is that the pope was an impartial arbiter. Alexander VI's Spanish origins and political ties complicate any claim of neutrality. The bull of 1493 leaned toward Spain. John's rejection of that bull and his push for a bilateral treaty shows he did not see papal arbitration as neutral.

Also worth correcting: Columbus never claimed a continent. He described islands throughout his early voyages. Much of the later dispute and the eventual Portuguese settlement in Brazil came from the geography of the line, not from an explicit plan. The messy outcome fits weak legal frameworks, not a clear masterstroke.

A professional caveat: historians still debate how much John actually intended to claim Brazil. Some argue the westward move of the line to 370 leagues was designed precisely to put Brazil — or lands near it — inside the Portuguese zone. Others argue John was focused on defending Portugal's African and Atlantic position and gained Brazil essentially by geographic accident. The evidence does not settle the question cleanly.

Practical actions: how this legal architecture worked in practice

If you were advising a European crown in the 1490s, the concrete steps would look like this.

Secure a papal bull first. Without one, you had no legal claim in the eyes of the Church. Portugal used this route repeatedly. Negotiate a bilateral treaty with neighboring powers. The Treaty of Alcáçovas set the template: define spheres by direction and named landmarks, not coordinates. Establish actual occupation. Prior discovery and sustained presence were evidence of possession. Portugal's Azores, Madeira, Cape Verde, and African settlements all served as proof. Build a legal record. Keep charters, charters of exploration, and bulls in a formal archive. Portugal's chancery did this systematically. Negotiate the demarcation line at the highest diplomatic level. When the 1493 bull favored Spain, John negotiated directly with Spain, not with Rome. Verify geography before contesting. Push for accurate surveys. The lack of reliable longitude measurement was a real weakness, and the 1494 line was a compromise made under uncertainty.

The threshold that mattered: a single line, drawn at either 100 leagues or 370 leagues west of a fixed cape, divided two empires. A difference of about 270 leagues changed the fate of a continent. This is why precise longitudes were not formally measured with modern accuracy until later centuries — and why the line was always somewhat theoretical.

Limitations of what the record can tell us

The record of King John's motivations comes largely from diplomatic correspondence and later chronicles, not from anything John wrote explicitly about intent. His case has to be reconstructed from the treaties, the bulls, and the negotiations, not quoted directly. That limits how far we can say "John believed X" with certainty.

The legal frameworks themselves were unstable. Papal bulls could be overridden by later popes or by bilateral treaties. The Treaty of Tordesillas was famously challenged by other European powers, and it was largely because it rested on papal and bilateral authority rather than on a multilateral agreement. Its enforcement depended on whether other states accepted it, and many eventually did not.

Finally, the historical record reflects what courts, chanceries, and chroniclers preserved. Much of the geographic reasoning was verbal and depended on maps that have not survived. Any account of what King John believed is therefore an interpretation grounded in the surviving legal and diplomatic record, not a direct window into his mind.

This is general educational information about historical legal claims, not legal advice. Readers seeking legal or historical interpretation for a specific purpose should consult a qualified historian or legal professional.