Originally Posted by WarChild

Sometimes, when you have dogs of the same sex together, they can't figure out a dominate order amongst themselves. In my case, the younger dog wants to be the alpha female, and the older dog isn't going to give up without a fight. This happens less often with males as they seem to be better at working out who's in charge through rough play and "competitions". When it happens with two females, called bitch fighting, it's almost always worse. The dogs will fight until one is killed and there's very, very little that can be done to modify this behavior.