What motivates warfare in a intergalactic civilization setting? It's always been a bit of problem for me. Like, why bother? If there's technology which allows people to travel relatively effortlessly across the galaxy (or galaxies!), fighting for resources does not sound plausible. Full-on interstellar war appears to be something that consumes more resources than could be gained by waging it, at least if the human scale is similar to ours.

Moral issues are a more … sensible? reason for such a troublesome endeavor. But what would be a disagreement worth killing and dying in an intergalactic scale? An evil overlord that just evilly wants to lord over all universe is an obvious solution. But it gets boring fast, and also it takes some convincing to accept 100% evil evillers whose only motivation is being evil. Monomaniac cult with a strong urge to convert or destroy non-believers falls into the same category as evil overlords. Both of those, if they really are just out to destroy everyone no compromise, not that different from a grey goo plague situation. It has to be dealt with but… ehh, there's just not enough noble tragedy. The enemy should ideally not be just plain stupid and evil.

Anecdotal space war causes
> Star Wars, natural Forces make some folks go Dark Side, can they even be held morally responsible for their actions? (btw it somewhat bothers me about new Star Wars is that we have Finn to prove New Order stormtroopers are individuals and can be rehabilitated, and yet no-one – not even Finn – considers them humans under their helmets. I'd think he'd like to capture and anti-brainwash some of them or at lest try some form of propaganda).
> Ancillary Sword, Ann Leckie. This is interesting bc the protagonists are the monomaniac cult. They feel obligated to subjucate everyone.
> The Three-Body Problem, Cixin Liu. Resources are scarce, so war makes sense and does not require philosophical arguments.
> Commonwealth Saga, Peter F Hamilton. The enemy is an expansionist cult of one (and easily my favourite character of the series).
> Surface Pattern, Iain M Banks. Severe philosophical differences, but not satisfactory. I adore Banks but the pro-Hell enemy side is definitely just plain evil and stupid.

For a really fascinating space war story there should be a disagreement, whose both sides are somewhat relatable AND such a far-reaching and serious issue it would motivate people to do terrible things and put themselves into danger. And  think I've just bumped onto one! It's a philosophical issue, both sides make sense, and it can't be left unresolved. Perfect.

The big question "Is it morally imperative to erase suffering to the point of wiping out wild animals/wild nature?". I fall quite definitely in the camp of "life wants to live and self-determination of even non-sapient creatures is more important than erasing suffering", but I can totally follow the arguments of the other side as well.

Even if the nature wipe-out scenario does not concern me personally, I'd still take up arms to oppose it. And it's not an issue one can really be lukewarm about, should it ever come to be practical to actually end all mindless suffering. Really a good reason to get involved and gather armies and shed blood and explode suns etc! I'm so not going to write an intergalactic war saga, but if I were...