Before starting of the movie "Apocalypto" it shows the quote:
A great civilization is not conquered from without until it has destroyed itself from within. - Will Durant
How is it related to the movie itself?
Well, I believe that this is attributed to many of the great civilizations decaying from within that contributed to the eventual fall of said civilization. There is a lot of evidence that the Maya empire was near the brink of collapse before the Spanish came (due to famine, overpopulation in specific cities, fighting among social classes, etc.). You can see many of these factors throughout the movie, not to mention the actual "harvesting" of people used for sacrifices that many "Spiritual Leaders" thought could help out these situations.