In the historian William H. Prescott's incredible book History of the Conquest of Mexico.
Prescott made it clear that "conquest for the glory of God" was a load of hokum. Cortes was after Aztec gold. But the "glory of God" stuff made a great politically correct excuse. Prescott covered the same theme with different characters in his equally incredible History of the Conquest of Peru.
Interesting that Prescott noted BOTH the Aztecs and Spaniards launched crusades/holy wars for their very different religions:
The tutelary diety of the Aztecs was the god of war... The soldier, who fell in battle, was transported at once to the regions of ineffable bliss... Every war, therefore, became a crusade; and the warrior, animated by a religious enthusiasm, like that of the ... Christian crusader ... courted... the imperishable crown of martyrdom. Thus we find the same impulse acting in the most opposite quarters of the globe... each earnestly invoking the holy name of religion in the perpetuation of human butchery.
Nobody reads Prescott any more...which is probably why I found his books at a yard sale for twenty-five cents each. It's a shame. He was about 150 years ahead of his time in writing popular history - i.e., not a dry drone of names and dates, but history that jumped right off the page and grabbed you by the throat. (His books are still in print and available at the South American River.)
Prescott also had an incredible personal story. During a food fight at Harvard, he was hit in the eye by a piece of bread. This blinded one eye and weakened the other, so that he was almost totally blind while still a young man. He died in 1859.
/pedant