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In June 1494, in the Castilian town of Tordesillas, Spain and Portugal signed one of the most astonishing treaties ever written: a vertical line drawn 370 leagues west of the Cape Verde islands split the planet in two. Whatever appeared west of it would be Castilian; everything east, Portuguese.
They didn't yet know what exactly lay across the Atlantic. They divided up oceans nobody had sailed and continents nobody had set foot on, backed by the bulls Pope Alexander VI had signed the year before.
The line has consequences that last to this day: the coast of South America bulges eastward and fell on the Portuguese side. That is why Brazil speaks Portuguese while its neighbours speak Spanish.
The rest of Europe was not amused. Francis I of France is credited with the best comeback: he asked to see the clause in Adam's will that left the world to Spain and Portugal.