Spain is the short answer, but it isn't entirely correct.
Cortes took off on his own, seeking to conquer the Aztecs without orders to do so. Perhaps the fact that he was facing some troubles and possible arrest motivated him to do so.
Had things not worked out, Hernan would have entered the history books in a very different manner, as it were however, he became a hero to the Spanish.
I doubt the Aztecs viewed him as kindly.