What are the conditions which must be met before the aliens invade your base? Can it happen again?
When I got raided by the aliens, this was my current story progress:
Date: July 8th. Difficulty: Normal. Alien Storyline: I raided the alien base some weeks ago and researched the hyperwave beacon, but I hadn't start building the hyperwave relay yet. Exalt Storyline: Two missions ago I completed the third covert ops extraction mission.