In one of their multitude of inter-nation conflicts; two cities were consumed in nuclear fire. On any sane world; these areas would be abandoned and the area around them a quarantine zone; the humans obviously not being fully sane, rebuilt and flourish.

This is related to the fact that humans along with most of the inhabitants of Sol3, are highly resilient to damage; of all kinds.

Humans reproduce slowly; but even though they generally reproduce not far above replacement rate, their resilience means their population still grows at an exponential rate. And on top of that; their advances in medical technology are on the verge of massive life extension. Soon they will have to move out into space; the galaxy better get ready for them.