Vast Swathes of Burnt Soil
It was relatively simple to keep the Empire happy: you just had to give them everything they asked for.
They had museums filled with mysterious weapons named after ancient, phantomlike civilisations. Names no-one could really pronounce anymore. They said: when the Empire was through with you, the only thing left would be that machine, with your culture’s name on it.
If another civilisations stood up to them, they would start fashioning a new weapon, all brass and glass and incredible heat, intending to name if after the affronting culture. They tended not to get far before they received formal apologies.