The Spanish and Portuguese dominated the first century of exploration, conquest and colonization in the Americas for many reasons. Spain and Portugal had a nearly eight-century long tradition of warfare from fighting the Reconquista against the Muslim people they called the Moors, and were prepared for further battle for glory and religion. The Portuguese also had a maritime tradition, which was how Columbus learned his trade.
But the two Catholic nations also had a papal charter. The Spanish and Portuguese were granted a license by the Pope in a Papal Bull of 1493 which led to the Treaty of Tordesillas that awarded all the territory east of the 47th meridian to Portugal and everything west of it to Spain. The Pope (Alexander VI) who made this deal was a Spaniard named Rodrigo Borja – his reign was known for nepotism and corruption, and, despite a pledge of clerical celibacy, he was the father of the famous Italian political family known for poisoning their enemies, the Borgias. The Uruguayan journalist Eduardo Galeano said of him, in a history called Memory of Fire, “A short time has passed since Rodrigo Borgia, of Xátiva, Valencia, took the name Alexander VI. Not a year ago yet since the day he bought for cash the seven votes he was short in the Sacred College, and could change a cardinal’s purple for the ermine cape of the supreme pontiff.”
The Portuguese received the east, because they were already establishing colonies on the east and west coasts of Africa. The Spanish were assigned the mostly-unknown west, which turned out to be bigger than anyone had expected. The Pope granted nothing to any other European kingdom. This type of corruption at the Vatican helped motivate reformers like Martin Luther toward what became the Protestant Reformation.
My reading of a sixteenth-century translation of the Papal Bull that is contained in American History told by Contemporaries: