More impressive were the number of natives living in absolute squalor in the area. It's not often that you see a culture regress.
It may not be regression. The squalor and poverty may be an improvement. Empire building on slavery and human sacrifice is not necessarily high culture.
I've just returned from Napoli, Itally. I'm afraid cultural regression is so uncommon. In the case of Napoli, I don't think it's recoverd from WWII.