Complex sentence: In 1526, Hernán Cortés writes in one of his letters of the northern Chichimec tribes who were not as civilized as the Aztecs he had conquered, but commented that they might be enslaved and used to work in the mines.
Split into simpler sentences: In 1526, Hernán Cortés writes in one of his letters of the northern Chichimec tribes, who were not as civilized as the Aztecs he had conquered. He commented that they might be enslaved and used to work in the mines.