Imagine this scenario:
You are an Azari, a native of the planet Azar. Your people have developed a global civilization with technology comparable to that of mid 20th Century humans on Earth. One day, dozens of alien spaceships appear in the skies above your world and then land, disgorging thousands of alien troops and tens of thousands of alien settlers. The aliens tell you they are Marlos and they claim to OWN your world, having bought it from their neighbors the Carlics a few years earlier. You remember having contact with the Carlics a century ago and even trading with them, but you had no idea they claimed to own your world, let alone that they had “sold” it to another interstellar power. So you reject the Marlos’ claim and attempt to repel the invaders. But the Marlos use their superior technology to quickly defeat you! Most of your people are exterminated and the few survivors are forced into concentration camps of poor land where they can only survive by farming, while the Marlos take over the richest land and nearly all the resources of the world your people evolved on! Your civilization, which could have become an interstellar power in its own right in a few thousand years, is instead broken forever.