In the case of that sentence, "rages on" means that the war continues. It also implies that the war is "raging," which is just used to emphasize how violently it is continuing. The writer is trying to paint a picture in your mind of anger and violence when he or she uses that word, instead of just saying "the war continued." Hope that helps!

Maybe some wars "rage" and others don't, because of different things about them. I don't want to think about war now though.