Sometimes, a force comes that simply utterly annihilates and obliterates what came before it. More often, it is a slow and gradual process as a status quo adapts and readapts to a new balance of powers. Other times, however, it can be a single, overwhelming force that truly is the stuff of legend and nightmare alike.
Destruction events can provide a clean slate, and not just in human history. Mass extinctions in Earth history usher in new types of dominant life, from dinosaurs and the age of mammals. Native Americans were nearly annihilated by European diseases, while colonists would swoop in to seize the freed real estate.
The destruction occurs when the ability of a system to adjust is overwhelmed by its ability to respond and reform, when the body fails to the pathogen. From barbarian invasions to plague biology, natural selection tends to favor the adaptive. While more specialized animals (the fastest, the smartest, the strongest, etc.) may die off, the common types endure. Compare machinery that can be built in caves to an over-complicated piece of crap. However, once the initial shock is gone, specialization occurs again. Such is a natural process of biology and economics alike. The lesson, however, is to have backups in case the over-specialized and delicate things fail. Because they will.