merrymouse, I suspect that has generally happened only where a larger power used the opportunity to invade, e.g. Roman empire invades kingdom where a civil war is being fought over who gets the throne and crushes everyone into submission.
Even in the modern era I am struggling to think of more than one example of anyone intervening successfully, certainly on a short or medium term basis: Nigeria (Biafra), Angola, Cote d'Ivoire, Mozambique, Somalia, Western Sahara, Congo, Chad, Rwanda, all no, and that's just in Africa. The UN is actually doing well in Congo and Somalia apparently, but it is a bitterly long haul and there will have to be a long political process even if there is military victory. France has intervened in Mali, successfully for now, but hopes are not high for the future. The French will leave, and there is every prospect fighting will start again when they do.
One can be more optimistic about Libya though, but there the societal split was less entrenched. Ethiopia/Eritrea played itself out, as did Sudan/South Sudan, until the two sides negotiated.
Lebanon no, Irag and Afghanistan no, Yemen no.
Yugoslavia yes, I suppose, the peacekeeping plus bombing concentrated minds and lo and behold the warring sides were ready to talk at Dayton.