Optional background: (Contains spoilers for the Nausicaa of the Valley of the Wind books by Hayao Miyazaki!)
After the 'Seven Days of Fire,' a global war that ended all forms of industrial civilization when it was at its climax, civilization was reformed somewhat into the kingdom of Eftal which retained many of the technological wonders from before the event. Close to 700 years after the Seven Days of Fire, disagreements over the right to the throne and attempts to destroy the spreading toxic jungle that had appeared just after the apocalypse resulted in the kingdom's near-complete destruction.
The insects which reside in the jungle were enraged by the constant attacks on it, and a surge of insects-- including the king insects of the jungle, the Ohmu-- overtook the capital city of Eftal and nearby lands, becoming seedbeds for the fungus of the jungle. Those that survived this tidal wave of forest and insect (called a Daikaisho) either became a tribe of mercenaries or entered the forest which they call their home, surviving off of a symbiotic relationship with the plants and insects that live there.
Based on Hayao Miyazaki's Nausicaa of the Valley of the Wind.