Dr. Dawn is correct – you must get the second dog neutered, and even then you need to keep these dogs apart (so they can’t even see each other!) for several weeks afterward.  This gives time for the testosterone to work its way out of the new neutered dog’s system. Then, get the help of a professional behaviorist to reintroduce these two dogs – don’t attempt it yourself because they now have a history of aggression and will associate this aggression with each other.  You need someone that can read the tiny signs a dog gives when they are uncomfortable… Read more »