Remember during world war two when 3 million Soviets defected to Germany. Anybody except the Arians, or anybody except the Americans, Egyptians, or Germans were scheduled for eventual extermination.
We can remember any war when people were switching sides. It is barely a "migration", don't you think?
Why would they side with their exterminators.
Good question. Maybe they hoped that Germans are highly cultural nation and won't do all those atrocities. Or they hoped they could get something out if it - how you can explain people from various nations serving in SS or other German units, and who were volunteers?
Adn what about those who defected only to defect again and return to their original side, because it was the only way to escape German POW camps?
When you are starving or in need of money as long as they aren't persecuting or trying to kill you it can look better to be in another faction as long as their is no anti policy this is going to happen the question is how much.
Yeah, sure "come to us and we turn you to canned food".
This might not be an option except everyone starts out on Terran worlds.
Like I said, I still do not exactly understand that interspecies migration, especially in case of certain species. This is where I like Endless Space more, where Cravers couldn't be at peace with anyone.