[Sardia]

I have a question regarding capturing planets. Sometimes, I send troops down and they surrender right away, without the ensuing militia vs troop battle at the end of turn. I managed to trigger it a few times by bombarding the planet while MY troops are still on the planet. However, I have gotten them to surrender the planet without a fight. Care to explain the rules regarding this?

Does it involve reducing the population? Or is there a "fear" factor? I do enjoy getting planets, and not waiting a turn is quite helpful.