The Treaty of Tordesillas and the Papal Division of Overseas Lands turn a distant European agreement into a direct challenge to Japan’s independence. In Blackthorne’s account, papal lines do more than divide unknown territory between Spain and Portugal: they authorize claims to lands already inhabited and connect conquest to the spread of Catholicism. When he explains that Japan has been assigned to Portugal, the map becomes evidence in a political confrontation with Toranaga.

Background: Secrecy and the Stakes of Navigation

In Blackthorne’s world, a claim across the oceans had to be backed by the ability to reach and control the places it named. Existing charts could be too vague to guide a voyage, and there was no way to determine longitude. The practical answer was a rutter: a pilot’s record of courses, depths, currents, winds, storms, safe anchorages, and hazards. Such books offered detailed knowledge of how to reach a port and return, but they could contain errors, even deliberate ones, and a pilot could not be certain of them without making the voyage.

The English, Dutch, and French had rutters for their own waters; routes elsewhere had been sailed principally by Spanish and Portuguese captains. Spain and Portugal treated their rutters as national secrets, guarding routes to the New World, through the Strait of Magellan and around the Cape of Good Hope, and onward to Asia. The secrecy of those navigational records gives the treaty’s mapped divisions a material counterpart: a line could express a claim, while ships and closely held knowledge shaped who could travel the route.

The Papal Lines and the Imperial Claims

Blackthorne explains the division to Mariko and Toranaga as a sequence of papal decisions. Pope Alexander VI established an initial line of demarcation in 1493. Pope Julius II sanctioned changes connected with the Treaty of Tordesillas, signed by Spain and Portugal in 1494. Blackthorne describes that treaty as dividing the New World between the two kingdoms. On the map, he traces the division through Brazil: Portugal claims the territory east of the line, while Spain claims the lands west of it.

He supports the abstract division with examples of what such claims meant in practice. Pedro Cabral reached Brazil in 1500, after which Portugal claimed its gold and silver and displaced its existing culture and rulers. Blackthorne says Spain claimed much of the rest of the Americas, including Mexico and Peru, and describes the destruction of the Inca nations, the enslavement of hundreds of thousands, and the extraction of wealth. In his account, conquest, the replacement of local government, and the arrival of priests are parts of the same process: conversion and local rivalries help make a realm vulnerable to annexation.

The later Treaty of Saragossa extends the argument from the Americas to Asia. Blackthorne says Pope Clement VII sanctioned it in 1529, drawing a second line of longitude across the tip of southern Japan. He presents that line as granting Portugal exclusive rights over Japan, China, and Africa, with exploitation justified in return for spreading Catholicism. The distinction matters: Tordesillas is introduced as the division of the New World, while Saragossa supplies the papal division Blackthorne says reaches Japan.

Blackthorne’s Disclosure in Japan

Blackthorne reveals the claims while explaining that he had been piloting a ship on an attempt to find a northeast passage. He tells Toranaga that, under the European agreements, Japan officially belongs to Portugal. Mariko rejects the assertion at first as nonsense. Toranaga laughs at the premise and observes that he could just as readily divide the heavens between himself and the Emperor of China. His response exposes the absurdity, from a Japanese perspective, of a foreign authority assigning ownership of his country without its consent.

Blackthorne insists that the claims are written into legal documents. He traces the line on a map and describes the terms as giving Spain and Portugal a right to claim lands discovered by their subjects, remove existing governments, and impose Catholic rule. Mariko grasps the political significance quickly, but the news unsettles her: she has to translate a claim that appears to contradict Japan’s own understanding of its sovereignty. Toranaga grows irritated by the delay while she conveys Blackthorne’s explanation. Her reaction shifts from disbelief to turmoil, while his account gives Toranaga a framework for understanding the religious and commercial power behind the Portuguese presence.

Sovereignty, Trade, and the Limits of the Claim

The scene places papal authority beside Toranaga’s insistence on Japanese jurisdiction. He tells Blackthorne that foreigners are in Japan only by permission and that public disorder or quarrels will be punished under Japanese law. The contrast is sharp: Blackthorne describes European rulers and the papacy assigning overseas territory, while Toranaga asserts that foreign status does not displace the laws of the country where those foreigners stand.

The balance is not simply a matter of competing maps. Blackthorne’s account of the papal division unfolds in a setting where Portuguese pilots have an established role in Japanese ocean travel: he explains that the Taikō had required the few Japanese ocean-going ships to carry Portuguese pilots. Elsewhere, Toranaga draws a boundary around foreign military action, rejecting attacks by one foreign nation on another on Japanese soil while distinguishing those from action against an enemy of the Emperor at sea. Together, these details frame the treaty not as settled ownership but as a claim whose force depends on navigation, trade, religious influence, and the political choices of Japanese rulers. Its decisive significance in the story is the knowledge it gives Toranaga: the danger is not only that Portugal trades in Japan, but that European religious and legal arguments can present control of Japan itself as an entitlement.