Greetings!
CEO Brad Wardell talks all about Invasions in his latest dev diary for Galactic Civilizations III: Crusade. He shares some details about how the system works - how to defend your planets, how to strategically choose where to attack other planets, how tech trees affect the potential success of your invasion, and more.
To read about it all in detail, check out the post on our forums here:
https://forums.stardock.net/482436/CRUSADE-DIARY-7-Conquest