conquest: I want what you have, even if it's just a populace that will pay me taxes and tribute to fund more war.
reputation: If you have nothing I want I'll run you into the ground anyway just to prove I can so the other countries will be scared of me.
rivalry: My cousin rules you and I hate that guy. This will also show all my other ruler cousins in other countries that I can kick their butts too.
assassination: A minor rebel group has successfully assassinated your/our ruler, and now all the other countries smell war so they're buffing up and making alliances. You're small, so you're probably toast.
imperialism/racism/social Darwinism: We're clearly the superior race- you're practically inhuman anyway- so we're just going to rub you out and take what we want.
revenge: You started it by being a show off and/or attacking us when we weren't looking or when we were weak. Maybe you even took a chunk of land or a national treasure from us, so we're taking it back by force.
independence: The two big countries once split a smaller country in half and now share it. The citizens of that smaller country never agreed to this and want independence again. They lose of course, but not until the two bigger countries escalate the matter.
science/technology: So, we like have this new weapon and want to test it out. Just hold still for a second...
manifest destiny/"God thinks we should": It was prophesied, written, or we just decided that God wants us to do this. Your god sucks anyway, so move it.
MONEY, MONEY, MONEY: Like five rich people including two politicians want some more money. They start a war, everyone dies, they go home with three times as much dough as they started out with. Their descendants will probably do the same thing in a couple decades...or tomorrow.
(Found on tumblr)