Blam, I do believe that smaller impacts are responsible for decline of ancient civilization as well as migrations patterns. This is on top of volcanic eruptions, earthquakes, and climate changes.
Nature has shaped the course of civilizations more extensively than political or military events. This is indeed something many scholars refuse to confront because they staked so much of their credibility on the rise and fall of civilization mostly by human events.
Especially in ancient times, what human could do is puny in comparison with what nature could unleash. It is even true today. We can just take care of minor natural events. However, we are still at the mercy of major natural events.