The king of the Portuguese discovered Spain was in charge of the new world trips, and he was not happy about the matter. He then decides to write a letter to Spain, threatening the catholic faiths. They were afraid and decided to inform the pope. This, therefore, created a line between the nations. In a manner of avoiding wars, the Spanish were, however, likely to lose.
Finally, the Portuguese were not happy with the division because it barred them from getting to India, so they decided to negotiate through the catholic monarch.
