1 Iki Piki: I didn’t know Earth had nationalistic space exploration.
2 Paris: Oh yes. Spain and Portugal in particular were very competitive, until they signed the Treaty of Taurus, dividing the entire Galaxy between them.
3 Paris: They agreed that everything west of galactic longitude 370 light years west of Chi Virginis was Spanish, and the rest of the uncolonised Galaxy belonged to Portugal.
4 Iki Piki: What did the other countries think?
4 Paris: ¿Por qué no ninguno de los dos?

Alternate punchline (replacing both Iki Piki and Paris in panel 4):

4 Paris: This allowed them to stop fighting each other, so they could focus on peacefully enslaving half the Galaxy each.

The Treaty of Taurus is a space analogue of the Treaty of Tordesillas, which was a real treaty signed in 1494 between Spain (or more historically accurately the Crown of Castile) and Portugal, which agreed that all newly discovered lands west of a meridian 370 leagues west of the Cape Verde islands belonged to Spain/Castile, and all territory east of that line belonged to Portugal.

Notably, this gave the first colonies in what would later become Brazil to the Portuguese, while the islands of the Caribbean, the rest of South America, and all of North America were rightfully Spanish territory. This might sound like a good deal for the Spanish and a bum steer for the Portuguese, but the entire continents of Africa and Asia were also east of the line, leading to the Portuguese possession of territories in Morocco, Guinea-Bissau, Angola, Mozambique, the Maldives, Bom Bahia, Goa, Kalikut, Sri Lanka, the Moluccas islands, and Macau among others.

A problem arose when the two empires realised they could successfully sail around the world. Portugal figured that as long as they could keep sailing east, they could start new colonies. But in 1521, Ferdinand Magellan's fleet on its historic circumnavigation of the globe reached the Moluccas by sailing west from the Americas. The Spanish then realised that the Portuguese had sailed more than halfway around the world east of the Tordesillas Treaty line. So, of course, they argued that the treaty had actually divided the entire world into two equal halves, and that the Moluccas—incredibly valuable for the spice trade—should in fact belong to Spain.

Charles V of Spain sent an expedition to colonise Moluccan islands. They established a fort on an island next to one already occupied by Portugal, leading to inevitable conflict and fighting.

To resolve the issue, in 1524 the competing kingdoms organised a joint conference. Each kingdom appointed a committee of three three astronomers and cartographers, three pilots, and three mathematicians to argue their case. The committees actually agreed that the world should be divided into two equal pieces, one half for Portugal and the other for Spain. The disagreement was where exactly the meridian line opposite the Tordesillas line lay.

There were a few problems. Firstly, the Treaty of Tordesillas didn't specify exactly where its dividing line was. It didn't say which point in the Cape Verde Islands the 370 leagues was to be measured from, nor did it specify the length of a league in a time when measures were far from standardised and different people used leagues defined in different ways, based on a wide variety of archaic Roman and Greek units. Secondly, maps of the world in this time were still inaccurate and highly variable. In particular, there was no good way to determine longitude accurately.^[1] So the Portuguese presented maps that showed the Moluccas in their half of the world, while the Spanish presented maps that showed the Moluccas in their half of the world. Of course.

The conference never reached an agreement. In 1525, with the question still unresolved, King John III of Portugal married Catherine of Austria, who was Charles V's sister, and then in 1526 Charles V married Isabella of Portugal, the sister of King John III. Now a cosy crossbred royal family, Spain and Portugal became allies. Wanting to avoid conflict with his new double-bestie-brother-in-law, and also slightly hamstrung by the argument that by all rights Spain would have to transport goods to and from the Moluccas via America rather than via Africa, which seemed virtually impossible, Charles V agreed to set the dividing line on the far side of the world to a location 297.5 leagues east of the Moluccas, thus ceding all of the islands to Portugal. This agreement was formalised in the Treaty of Zaragoza, signed in 1529.

The Zaragoza line meant that Portugal got most of Australia, New Guinea, and Japan, while Spain could claim the smaller eastern portion of each of these lands, as well as New Zealand and most of the islands of the Pacific. Or they would have if they had discovered any of them in time. Because although these treaties settled colonial rights between Portugal and Spain, all of the other colonial powers—the English, French, and Dutch—simply ignored it and grabbed whatever land they could find. And of course the native populations that the European powers found already living there had pretty much no say at all.

You've just got to admire the sheer brazen audacity of the original Treaty of Tordesillas. Basically two dudes getting together and saying, "We'll divide the entire world between us, and sucks to be anybody else."

[1] And there wouldn't be until the late 18th century. See the second half of #3341 for a brief overview.