Savage people recovering a savage land. When the civilized people are all killed or forced to flee, the area will return to its state of nature: anarchy, cyclical famine, ignorance. Predictable. Inevitable.
Read the “Congo” series at the blog “Unamusement Park”, including a linked article from Time magazine circa the late Fifties-early Sixties describing a flourishing economy at that time. The “Congo” series descrines exactly what you say-return to nature, that means both environmental and human, including cannibalism, diseases which had been eradicated or controlled, uncontrolled rapes (including infants), savage horrific mutilations, return to wotch doctor practices, etc. It’s like civilisation never touched the place.