Actually, body language does convey a lot. If you see someone angry, can't you sense them coming? Dogs can do the same.
There are several different tones in a dog's vocal abilities, and they can perceive what we can not, too, so what do they convey that we don't even catch?
It's adapted for dogs to understand, just like a humans' communication is adapted for humans. It's not hard to think there could be vast networks to many animals' communication that we don't see, simply because they mean nothing to us (and the same is said of ours to them).
And faradon also brings up a point, too. I mean, how often do animals invade each others' territories and decimate any who oppose? Elephants have been known to throw rhinoceri for the littlest of things, or just because...