This is the second game where the Quameno simply invaded one of my planets without declaring war or anything and then go about business as if nothing happened.
My relations with them are always decent when this happens, but needless to say, I can't let them get away with it and I take a reputation hit if I declare war to take the planet back. If I just invade the colony Quameno-style without declaring war, they get offended and declare war on me.
So I take it that when someone invades a planet, it doesn't instantly trigger war, but rather it's up to the invaded empire to explicitly declare war?
They do this to me constantly as well. I was playing along and ordered the colonization of several systems between their sphere of influence and mine. A few turns later I take a look down there and they now have colonies where I should have them.
This seems odd to me, so I start to pay more attention to my colonies on that border and I catch them invading one. I instantly went to war over it and my reputation hit was so severe that several other races declared war on me as well.
Needless to say, I didn't last long that game. Now, when I have the Quameno as opponents, I try and force them to declare war .. let them take the rep hit.