League of Free Cadorie by Romana Drew

A different kind of alien invasion story. An isolated valley in the middle of a sparsely populated continent is the perfect place to build a mine and foundry to produce raw materials for an orbiting shipyard. The little, furry locals are too primitive to even notice. Everything went fine until the natives collapsed the mine and blew up a few buildings. The League of Free Cadorie pits a gentle agrarian people against aliens bent on striping their natural resources.