Preface: Welcome to this quick recap of the history of immigration of US. Celebrating 250 years of USA.

On the seventh of June, 1494, in a small Castilian town north of Medina del Campo, six men and two teams of geographers finished dividing the surface of the earth in half.

The Treaty of Tordesillas drew a line from pole to pole through the Atlantic, 370 leagues west of the Cape Verde Islands — roughly 46 degrees west, though nobody at the table could have fixed that longitude at sea if their lives depended on it. Everything east of the line belonged to Portugal. Everything west belonged to Castile. Ships were forbidden to sail into the other party’s half, with one exception: Castilian vessels could cross Portuguese water in a straight line, so long as they kept going west. The two crowns also agreed not to run to the pope for help against each other — only to ask him, afterward, to bless what they had already decided.

It is usually described as a treaty between Spain and Portugal. To be exact, at the time Spain did not exist yet. Isabella and Ferdinand had wed in 1469, and the union that resulted was personal, not political — two crowns worn by two people who happened to share a bed and a set of enemies. Castile and Aragon kept their own parliaments, their own coinage, their own laws, their own customs houses on the border between them. An Aragonese merchant was a foreigner in Seville. Ferdinand’s name appears on the treaty because he was Isabella’s husband; the territory being divided was Castile’s and Castile’s alone.

Columbus had sailed under the Castilian crown, the Indies would be “legally” Castilian property for the next two hundred years, and Aragon — busy with Naples, facing the wrong ocean — was excluded from the New World it is now assumed to have discovered. Granada, the last Muslim kingdom on the peninsula, had surrendered twenty-nine months earlier. Navarre would not be annexed for another eighteen years. The country signing away half the planet was less than three years old and not yet a country.

Portugal, on the other hand, had been a continuously governed state since 1139. It finished its own reconquest in 1249, ran out of frontier, and turned to the water — Ceuta in 1415, then fifty years of grinding, incremental, state-funded work down the African coast under Henry the Navigator, a program that outlived the man who started it. Portugal built the instruments, the ships, the tables, the method of getting back. Castile spent those same decades in aristocratic civil war, with a king so weak history nicknamed him the Impotent (Enrique IV of Castile). It arrived at the ocean late, took one enormous bet on a Genoese (Christopher Columbus) with bad arithmetic, and won. Portugal didn’t miss the opportunity through timidity; it declined because its institutional expertise was good enough to detect a fraud. Castile eventually said yes in 1492 for reasons that had nothing to do with the geography being sound — Granada had just fallen, the crown had an army with nothing to do and a surplus of confidence, and the sum involved was small enough to lose.

1493 / Christopher Columbus

Columbus, returning in March 1493, was blown into Lisbon before he could reach Spain — so João II (João II of Portugal — reigned 1481 to 1495) heard the news first, from Columbus himself, and immediately claimed the discoveries were Portuguese. He had a decent case. The Treaty of Alcáçovas (1479) and the bull (a papal bull is a formal written decree issued by the pope — the heaviest instrument in the medieval Church’s paperwork) Aeterni regis (1481) had given Portugal everything south of the Canaries, and the 1481 bull is precisely what the later dispute was framed against.

Ferdinand and Isabella went straight to Rome. The pope was Alexander VI — Rodrigo Borgia, Valencian by birth, elected in 1492 with heavy Spanish backing and owing favors accordingly. Over the spring and summer of 1493 he issued a series of bulls, the key one being Inter caetera (May 1493), which drew a line 100 leagues west of the Azores and Cape Verde and granted Castile everything beyond it. A later bull that autumn went further still, arguably handing Castile a westward route to Asia itself.

João II refused it. He had a fleet, he implied he might use it, and he demanded direct talks instead of accepting a ruling from a pope he considered bought. The Castilians agreed — they had what they wanted and no appetite for a war. Both delegations met at Tordesillas in early June 1494.

June 1494 / Tordesillas

The papal line of 1493 had been drawn 100 leagues west of the Azores and Cape Verde. Portugal could not accept it, for two reasons.

The first was that the bull was ambiguous. It said the line ran “100 leagues west of any of the islands of the Azores or the Cape Verde islands.” But the Azores sit at 37 to 40 degrees north, and Cape Verde at 15 to 17 degrees north — more than two thousand kilometers apart. A single meridian could not run 100 leagues west of both. The bull left the choice of reference island open. Any careful Portuguese reading of it produced two possible lines several degrees of longitude apart.

The second reason was practical. A line only 100 leagues off Cape Verde ran through the water Portuguese ships needed for the volta do mar — the wide swing into the South Atlantic that any Cape-bound fleet had to make to catch the westerlies home. Bartolomeu Dias had rounded the Cape of Good Hope in 1488. Vasco da Gama would sail three years after Tordesillas. Portuguese pilots already knew the return route required a swing far into open water that the papal line had just given to Castile. This was the technical objection João II led with, and no Castilian pilot could deny it.

So Portugal came to the table asking for 370. Not 200, which would have covered the volta with margin. On a modern map, that puts the line at roughly 46°37’ west — a meridian that slices through the eastern bulge of South America. What the extra distance bought Portugal, in navigational terms, was a comfortable corridor for its Cape-bound fleets. Portugal may not have picked 370 just for navigation. They may have picked 370 because they already suspected there was land out there and wanted the line drawn far enough west to include it. Six years later, Cabral “accidentally” made landfall on a coast that turned out to fall right inside the zone the treaty had already given to Portugal. That’s either an astonishing coincidence, or evidence that Portugal knew what it was doing in 1494.

To trace the 370-league line on a modern atlas: it enters South American waters north of the Amazon delta, coming ashore just east of the modern city of Belém, which sat on the Castilian side by a whisker. It runs south through the eastern edge of the Amazon basin, just west of modern Brasília. It crosses the interior and the state of São Paulo — the city of São Paulo sits at 46°38’ west, essentially astride the line. It reaches the Atlantic near the town of Laguna, in Santa Catarina, where a stone marker still commemorates the landing point. Everything east of that trace was Portuguese. Everything west was Castilian.

In practice, Portuguese Brazil was a coastal strip. The great cities of colonial Brazil — Salvador, Recife, Fortaleza, Rio de Janeiro, and the whole seaboard south to Laguna — all sat east of the line, on the Portuguese side, with hundreds of kilometers of margin. This is the strip the Portuguese crown began to occupy in the 1530s under the capitanias hereditárias system. What was not Portuguese under Tordesillas is most of the country a modern reader would call Brazil: the Amazon basin from Manaus to the Andes, the vast interior of Mato Grosso, Goiás, Rondônia, Acre, and the Rio de la Plata basin. Portuguese Brazil in 1494 was perhaps a third of the modern country, arranged as a long thin arc along the sea. The rest, Portugal took later, without treaty authority, by simply settling it.

Six men signed. For Castile: Enrique Enríquez de Guzmán, Gutierre de Cárdenas, and Rodrigo Maldonado — the royal Mayordomo Mayor, the Contador Mayor and Comendador Mayor of Santiago, and a jurist of the royal council known as Doctor Talavera. For Portugal: Ruy de Sousa, his son João de Sousa, and the jurist Arias de Almadana. Two teams of geographers sat behind them. The negotiations were technical, brief, and — for a document that would allocate two continents — remarkably one-sided in their outcome.

Technical, because what the two delegations were actually arguing about was not principle but measurement. Both sides had already conceded the concept of a north–south meridian dividing an ocean neither could see across; the question that took months to work through was where the meridian sat, in units nobody could agree on, referenced to islands nobody had precisely surveyed. The Iberian legua was not one length but several — the Portuguese league of about 5.5 kilometers ran shorter than the Castilian league of roughly 5.9, and the maritime league used at sea was different again. A number of “leagues” from Cape Verde meant one distance to a Portuguese pilot and a distinguishably different one to a Castilian, and the treaty text did not stipulate which league was meant. Which of the Cape Verde islands the count was to run from was also left open; when cosmographers were later dispatched to actually mark the line, the Castilians measured from San Antonio (the westernmost island) and the Portuguese from Sal (the easternmost), and the two teams never agreed on which one the treaty had meant. Behind the ambassadors sat two teams of geographers, cartographers, and pilots, working with astrolabes, portolan charts, and tables of solar declination, because the terms of the document required judgment on questions of practical navigation that a diplomatic staff could not answer alone. The treaty is one of the first international agreements in European history whose technical annexes were doing more work than its clauses.

Brief, because from formal opening to signature the whole process took about three months. Preliminary contacts had begun in the closing months of 1493. Full negotiations opened at Medina del Campo in March 1494, continued there until 8 May, and were then moved fifty kilometers north to Tordesillas — reportedly to be closer to Isabella, who was recovering from an illness at the time — where they ran for a further month and a half before the final act was signed on 7 June. Three months, to redraw the world. By the standards of the century — the peace negotiations that ended the Italian Wars a generation later would run for decades — this is a fast document. It moved quickly because both sides wanted it to. Isabella and Ferdinand wanted the Portuguese threat off their western border so they could consolidate the Granada campaign and turn to Italy; João II wanted the Cape route protected before Vasco da Gama sailed. Neither side had time for a long negotiation, and neither side treated the specifics of the meridian as if they mattered as much as the fact of a signed agreement.

One-sided, because the concessions did not balance. The Portuguese delegation arrived with a number — 370 leagues — and left with that same number. There is no evidence in the surviving record that the Castilians tried to negotiate it down toward 200 or 250. What Castile received in return was a bundle of confirmations rather than substantive gains: Portuguese recognition of Castilian sovereignty over what Columbus had already reached, a withdrawal of João II’s threat of naval action, and a clause committing both crowns to keep the pope out of any future dispute — a small win against Alexander VI that mattered more in that specific moment than it would in the long run. What Portugal received was material and geographic: 270 leagues of additional westward zone, formal protection of the volta do mar, cover for the still-secret Cape route to India, and — as Cabral would demonstrate six years later — legal title to a coastline that João II may have already known was there. The composition of the two delegations helps explain how this outcome was reached. Portugal sent professionals: Ruy de Sousa had led embassies to the Kongo, his son had grown up in a diplomatic household, and Arias de Almadana was a jurist familiar with maritime law. Behind them stood a Portuguese cosmographic establishment that had spent eighty years mapping the Atlantic. Castile sent grandees: Enríquez was a household officer, Cárdenas a treasury official, Maldonado a jurist and rector of Salamanca. None of them were sailors. The Portuguese knew what the numbers meant; the Castilians knew what a signature meant. The asymmetry was not diplomatic incompetence on the Castilian side — Enríquez, Cárdenas, and Maldonado were formidable men, and the terms they accepted served Castile’s short-term interests — but it was a mismatch in geographic literacy that let the Portuguese set every technical parameter of the agreement.

July 1494 – 1498/ The immediate aftermath

Isabella and Ferdinand ratified on 2 July 1494. João II ratified at the convent of Setúbal on 5 September. Notably absent from the ratification calendar: the pope. Alexander VI, who had drawn the original line and taken the credit for dividing the world, was not asked to bless the new one. The two crowns had agreed at Tordesillas not to run to Rome — a clause aimed directly at a Borgia pope both sides now regarded as too useful to trust.

The ink dried and nothing changed. Isabella and Ferdinand went back to consolidating Granada and preparing for their Italian entanglements. João II turned back to preparing the Cape route to India. The Tordesillas line was, as a piece of legal geometry, agreed. As a piece of physical geography, nobody could locate it. And crucially, in July 1494 neither crown had actually reached anywhere the line would matter. Columbus had touched the Caribbean but not the American mainland. Portugal had rounded the Cape of Good Hope (Dias, 1488) but had not yet gone on to India. The line divided a world neither party had yet visited.

The treaty had specified that both crowns would send a joint expedition within ten months of signing — sailing out from Cape Verde with paired teams of cosmographers and pilots, to physically mark the meridian by taking latitude and longitude readings at sea and agreeing on a location. Cosmographers were named on both sides. Meetings were held. The expedition never sailed. Ten months came and went, then two years, then five, and the joint marking never happened. The reasons were partly political — João II died in October 1495 and was succeeded by his cousin Manuel I, and Castile was distracted by the death of Prince John in 1497 and dynastic turmoil — but the underlying reason was practical. Neither crown could actually locate the line, both crowns knew this, and both crowns had begun to realize that a vague line served them better than a precise one. If the meridian was somewhere between 44°W and 49°W depending on which league and which reference island you used, then Portugal could claim as much of the eastern bulge of South America as it could physically reach, and Castile could claim as much of the western hemisphere as it could physically reach, and neither had to concede anything to the other in advance. Ambiguity was a feature, not a bug.

Columbus made three more voyages after the treaty. His second (1493–1496) established the settlement at Hispaniola. His third (1498) reached the mainland of South America — the Paria peninsula in what is now Venezuela — which was the first time any European set foot on continental soil in the western hemisphere. His fourth (1502–1504) reached Central America. None of these landings ever came anywhere near the Tordesillas line. Columbus was operating hundreds, then thousands, of kilometers west of the meridian. His voyages did not test the treaty; they simply confirmed that whatever he was reaching was on the Castilian side of any line anyone could reasonably draw.

Portugal’s project was the opposite. Manuel I, on ascending in 1495, made the Cape route to India the crown’s priority. He commissioned Vasco da Gama in 1496 and dispatched him from Lisbon in July 1497 — three years after Tordesillas, one year after the joint marking expedition was supposed to have sailed. Da Gama executed the volta do mar for the first time on a scale anyone had attempted: swinging deep into the South Atlantic to catch the westerlies south, then around the Cape and up the East African coast, reaching Calicut on the Malabar coast of India in May 1498. He returned to Lisbon in September 1499 with a cargo of pepper and cinnamon that repaid the entire cost of the expedition many times over. This was what Tordesillas had really been about, from the Portuguese side. Da Gama’s voyage established that the eastern route to Asia worked, that Portugal could reach and hold it, and that the volta the treaty had been drafted to protect had been correctly identified as the load-bearing question. As long as the Tordesillas line sat far enough west to protect that swing, everything east of it was Portugal’s to develop.

One of the strangest features of this period is what Rome did, or rather didn’t do. Alexander VI, the pope who had drawn the original 100-league line in Inter caetera and who was still alive and reigning, was never asked to ratify the Tordesillas agreement. The two crowns had agreed at the negotiating table to keep the pope out of their bilateral arrangement, and they kept him out. Alexander died in 1503, replaced briefly by Pius III (twenty-six days) and then by Julius II. Julius, who had different priorities and a less compromising relationship with Iberian politics, finally ratified Tordesillas on 24 January 1506 — twelve years after the fact. By then, the world the treaty divided had already been substantially explored. Da Gama had reached India, Cabral had reached Brazil, Columbus had died (May 1506, four months after Julius signed), Amerigo Vespucci had explored the South American coast for both Portugal and Castile, and Portuguese fleets were operating in the Indian Ocean. The papal signature was, at that point, catching up to events.

What It Was Like Sailing in 1494

In 1494, a ship in mid-Atlantic could fix its latitude within a degree or two using the sun or the pole star. The method was straightforward. At noon, when the sun reached its highest point in the sky, the navigator measured its angle above the horizon using a mariner’s astrolabe — a heavy brass ring with a sighting bar, hung from a cord so it would settle vertical against the roll of the deck — or, in Portuguese practice by this decade, a wooden quadrant. He then consulted a table showing where the sun sat on that specific day of the year, adjusted for the season, and did some arithmetic. What came out was his latitude in degrees north or south of the equator. At night, in the northern hemisphere, he could do the same thing with the pole star, which sat almost directly above the North Pole and required no seasonal correction. The Portuguese crown had been publishing standardized solar declination tables — the Regimento do Astrolábio e do Quadrante — since around 1485, and any competent Portuguese pilot working the African coast knew how to use them. A degree of latitude is about 111 kilometers, so “within a degree or two” meant a pilot could place his ship within a few hundred kilometers north or south of where he thought he was. Not precise, but usable.

Longitude was a different problem entirely, and it had no solution. The navigator had essentially no way to know how far east or west he was, because nothing in the sky told him. Latitude works because the sun and stars sit at predictable heights that vary with your position north-south. But the earth turns, and the sun and stars move across the sky from east to west constantly — which means that measuring their position tells you what time it is where you are, but not where you are along the east-west axis. To convert time into east-west position you would need to compare local time to the time at a known reference point back home, and to do that you need a clock that keeps accurate time at sea. In 1494 no such clock existed. Mechanical clocks of the period ran on weights and gears that a rolling ship threw out of true within hours. Sundials were useless once you’d left port. The one instrument a navigator carried that measured time at all was a sandglass — usually a half-hour glass, turned by the ship’s boy at the sound of a bell — and while it was good enough to divide the watches of the crew, it was hopeless as a reference for planetary time.

What captains did instead was dead reckoning. The name is nautical shorthand for “deduced reckoning” — a method of calculating your position by starting from a known point and adding up everything you have done since. The navigator estimated his speed by watching the water go past the hull, judging it against experience; the mechanical chip log — a triangle of wood on a knotted rope, dropped astern and timed against a sandglass — would not come into common use until the 1570s. He estimated his direction from the compass, though the compass itself pointed to magnetic rather than true north, and the angle of variation between the two changed with location in ways that were only partly understood in 1494. He estimated elapsed time on each heading from the sandglass. Then he multiplied speed by time on each heading, plotted a small vector on a chart, connected it to the previous vector, and called the far end of the line his current position. He did this every few hours across a crossing that might last six weeks.

Every one of those inputs carried an error. The speed estimate was a guess, based on how fast the water looked; a pilot in a heavy following sea would overestimate, a pilot in a chop would underestimate. The compass reading was corrupted by magnetic variation the navigator could only roughly correct for, and by the iron in his own ship, which no one yet knew disturbed the needle. The sandglass ran fast in cold weather and slow in warm, and the ship’s boy who turned it was not always attentive. Each individual error was small — a knot too fast, half a degree off the true heading, a minute lost per hour on the glass — but they compounded across days and weeks. A ten percent error in speed and a five percent error in elapsed time, over four weeks at sea, could put a fleet three or four hundred kilometers away from where the pilot thought it was. In practice, dead reckoning got a competent navigator across the Atlantic — Columbus’s four voyages worked, more or less — but “worked” meant reaching the general vicinity of the target latitude and then sailing along it until land came in sight. It did not mean knowing, on any given day at sea, which side of an imaginary line running from pole to pole the ship was actually on.

1500–1506 / mapping the world that’s been divided

The most important of the follow-up voyages, from a cartographic point of view, was Gonçalo Coelho‘s expedition of 1501–1502, on which Amerigo Vespucci sailed as a pilot. The fleet left Lisbon in May 1501, reached the northeastern bulge of Brazil in August, and worked its way south along the coast for the next fifteen months. It named features it passed — Cabo de São Roque, Rio de São Francisco, the bay it entered on 1 January 1502 which it called Rio de Janeiro — and continued at least as far south as the Río de la Plata, possibly to the Patagonian coast beyond. When Coelho’s ships returned to Lisbon in September 1502, Portugal had, for the first time, a systematic survey of several thousand kilometers of the South American Atlantic coast, and Vespucci had the material for the letters he would soon publish, which would name the continent for him five years later. The empty western space on the charts had begun to acquire a shape.

That shape appears, for the first time, on the Cantino Planisphere. The map was drawn in Lisbon in the second half of 1502 by an anonymous Portuguese cartographer. It measures 220 by 105 centimeters — a large sheet, six pieces of parchment glued to canvas — and shows what the Portuguese crown then knew of the world: the West African coast, the Cape route to India, the Indian subcontinent, the Brazilian coast from Coelho’s survey, and the Corte-Real fragment of Newfoundland. A blue vertical line runs from pole to pole down the Atlantic, marked as the Tordesillas meridian, drawn to the Portuguese specification — that is, placed just west of the Brazilian coast, comfortably including Cabral’s landing on the Portuguese side. It is the earliest surviving map to show the treaty line as a physical feature of the world. The reason it survives, and the reason it is called the Cantino Planisphere, is that Alberto Cantino, an Italian agent of Duke Ercole I d’Este of Ferrara, bribed a Portuguese cartographer to copy the crown’s internal master chart and smuggled the copy to Italy in November of that year. He paid twelve gold ducats for it. Under Portuguese law, unauthorized copying of the master chart was a capital offense.

The master chart in question was the Padrão Real, the Portuguese crown’s secret official map of the world, maintained by the Casa da Índia and updated as each returning fleet reported its findings. Portugal treated it, and the geographic information on which it was based, as state secrets. Every pilot returning from a voyage was required to submit his log and charts; every mapmaker producing exportable versions worked from carefully controlled copies. This is the earliest sustained European example of geographic information as state property, and Portugal enforced it strictly for the next century. Cantino’s map is the copy that got out. What it reveals, incidentally, is that as of 1502 the Portuguese cartographers already understood that the Tordesillas line had an eastern counterpart on the other side of the world — the Cantino planisphere shows the earth as a coherent whole, and any competent viewer looking at it in Lisbon or Ferrara could see that a line running from pole to pole down the middle of the Atlantic must have a matching line running from pole to pole somewhere through the western Pacific. Nobody was yet arguing about where that eastern line ran. The islands it would eventually divide had not yet been reached from the west. But the geometry of the eventual dispute was, by 1502, visible on parchment.

The papacy, meanwhile, caught up. Alexander VI, the Borgia pope who had drawn the original 100-league line in 1493 and been kept out of the Tordesillas renegotiation the following year, died on 18 August 1503. His successor Pius III reigned for twenty-six days before dying himself. Julius II, elected 1 November 1503, ruled with different priorities and a less compromised relationship with Iberian politics, and in January 1506 he issued the bull Ea quae pro bono pacis formally ratifying the Treaty of Tordesillas. The date was 24 January 1506, twelve years after the two crowns had signed. By then, the world the treaty divided had begun to take on a visible shape: Portugal had reached India, Cabral had reached Brazil, Coelho had mapped the South American coast, the Corte-Reals had reached Newfoundland, and the Cantino Planisphere was already in Ferrara. Four months after Julius signed, on 20 May 1506, Christopher Columbus died in Valladolid, still insisting the islands he had reached were part of Asia. 🇺🇸