I actually think a planet with life already on it may be the worst candidate for territory. Sure, there's unlikely to be a plague that can affect them, but ther

I actually think a planet with life already on it may be the worst candidate for territory. Sure, there's unlikely to be a plague that can affect them, but there are millions of proteins and organic compounds on our planet that alien life has never been exposed to before. The odds are high that some substance that is commonplace on our planet has a toxic interaction with the aliens. Even if the aliens have a similar biology to our own (carbon based life with DNA, same cell structure and organelles, etc.) if they use a different chirality of an amino acid you could have tons of potential for toxic interactions. If that is the case, the only way to make the planet habitable is to wipe out all life on the planet, AND break down all of the toxic proteins, at which you're basically doing terraforming anyway. But yeah I agree that there's very little our planet has that an alien civilization would want. The only reasons I could see for war would be some sort of pre-emptive response ("some day they will be able to attack us, let's wipe them out before they can") or an ideological conflict. Maybe they have a religion/ideology/culture that sees us as needing to be destroyed. Maybe if they see us as a lesser race to be exterminated, like the Nazis. Or maybe we are evil in their morality. There's the sci-fi staple of seeing mankind as inherently violent and warlike, or polluting, but also more exotic things. Maybe they are immortal and don't reproduce, and see us and our potential for expansion as a sort of cosmic cancer. Maybe they are photosyntheic beings, and see anything that lives by consuming other life as a threat. But I think those are unlikely. At the very least I think that they wouldn't wipe us out on sight. If they did have strong opinions on religion or forms of government, it would likely take years of contact and dialogue to even figure out we were in disagreement. Even then diplomacy and diplomatic pressure would probably be used long before military force.