A Casus Belli for Interstellar War?
At the Volokh Conspiracy and Samizdata, there are discussions of what uncontroversial aspects of today's society will seem barbaric a century from now. Maybe we can get around our mundane prejudices by considering displacing the controversy in space instead of time. What uncontroversial aspects of human society will seem barbaric to extraterrestrials?
There is the obvious candidate of intelligent herbivores being horrified at eating meat. I once came up with the idea of intelligent autotrophs horrified at eating anything (also see Permanence by Karl Schroeder).
I recently thought of another possibility: What if we encounter intelligent single-celled creatures who think of multicellular creatures as horrible totalitarian regimes? What if we're invaded by beings seeking to liberate our blood cells?