Ontology is the branch of philosophy that starts with the question of what things are what, and gets more incomprehensible from there. Most branches of philosophy have the second part of the sentence, but ontology is still singled out for special dislike. I can sympathize with this position: making up categories without application is a spectacularly useless activity. But the thing that isn’t appreciated enough is that without ontology, ethics cannot be possible, because ethics must refer to entities. I’m going to argue that this justifies ontology.
Can you harm a nation? Is it wrong to harm a nation? Your nation? Before we can even ask the question, we need to know if ‘nation’ is an entity that can be harmed in a morally relevant sense. Can I harm a forest? We might say that it is possible to harm a forest, because we can identify better and worse forests (a forest that has been burned to the ground and salted is a worse forest, a forest with an invasive species that is taking over is a worse forest), and moving a forest to a worse state seems like a reasonable interpretation of harm.
Does this seem like a reasonable definition? Well, we can’t say that harm is necessarily morally bad: I can harm my computer (say, by opening “virus.exe”1). But that doesn’t seem unreasonable. Can I harm a quark? No, not really. How could I change a quark such that it is worse at being a quark? Harm seems restricted to complex multi-state systems. That seems reasonable.
So what is a nation? If you ask an international relations theorist, they’ll say it’s a state, or they might call it a ‘singular rational political actor under anarchic conditions.’ I think that the sociologists give a better answer here: a nation is a group of people connected by common consciousness, traditions, and origins. It seems like that’s the sort of thing that can be better or worse. If a group is happy, confident, growing, and trusting, that seems like a better group than one that is miserable, falling apart, and paranoid. So a nation can be harmed. Can it be harmed in a morally relevant way?
Do we care about groups? I say no. If there were no people, but there were groups, I wouldn’t care about harming them. If every French person and person of identifiable French descent died tomorrow, and in a thousand years some historian said something negative about the French, it’s hard to say that they are doing morally relevant harm. On the other hand, if I modify traditions such that all the individuals are happier but the group has split into two halves, it certainly seems like I have harmed the group. My intuitions say that I still have done a good thing. However, you may disagree. If so, why are nations complex systems that you care about differently than forests? Presumably there has to be some way of differentiating between them. Is the pattern of atoms in your computer deserving of moral value?
I’m not going to claim that what you think determines your ethical principles. But if you don’t think that groups can be better or worse, then the idea of betraying one won’t even be possible to you. Your ontological commitments. determine what types of ethics are thinkable.
Ontology also tries to address other problems: are there moral laws independent of humans? Are they discoverable? Does God exist?
As ever, one of the best advanced resources is the [Stanford Encyclopedia of Philosophy](http://plato.stanford.edu/entries/logic-ontology/#Ont). Good example of some ongoing debates in ontology are found in [Reism](http://plato.stanford.edu/entries/reism/) and [Ontological commitment](http://plato.stanford.edu/entries/ontological-commitment/)
1: Actually, I use linux. So a .exe file won’t affect my computer, and nobody would write a virus for my operating system anyway. But you get the point.