My dog was attacked and now he is aggressive to other dogs. How can I help him?

I have a Bull Mastiff/Bulldog mix named Chief. When I rescued him from the pound he was great with other dogs. In fact when I brought him home he immediately ran up and started playing with the other dog in the house (A Bull terrier). Enter the problem. The other dog was my sister's boyfriend's dog named Cali. And unbeknownst to me Cali had already had a history of dog on dog violence. Well we leave them in the backyard sometimes and occasionally when I brought them back inside they would have little nicks and cuts on each other. We figured it was a section of dilapidated fencing that they were scratching themselves on. We fixed the fencing only for the nicks and cuts to continue. Well finally one day I just happen to be looking out the backyard and I saw Cali attack Chief from behind. By the time I got to them Chief had a huge gash on his leg and he had Cali by the throat, though he wasn't biting deep enough to maim. I separated them and the next day, after extensive deliberation, my sister insisted that Cali leave the premises. (I turned out to be the one who had to take her to the shelter, and yes that sucked). Enter a couple months later I realized that Chief was now hyper-aggressive towards other dogs. To the point that he didn't even want to be near them. I've tried talking calmly to him and holding him when around other dogs but he just seems to be ignoring me. I know that in this situation it is usually an assertion problem, but the thing is he is extremely submissive at all other times, then another dog shows up and BAM he's a vicious mongrel. I've found articles saying that he needs to understand that I will protect him and that he may have lost trust in me but I don't know how to re-establish this trust. He is an amazing Dog and there is no way that I will ever get rid of him but this is becoming a problem as many of our visitors have dogs and I keep having to put Chief away when they visit. If anyone with experience has any good suggestions I would be much appreciative.

Send one through the head old yeller style.

If you got punched by the kid down the street, for no reason, at all, unprovoked, you'd be wary, or be aggressive towards the kid down the street, wouldn't you? Well it's the same for your dog?Your dog is basically shell-shocked? He's been attacked by another dog, and mentally, he's been scarred so he begins to associate all dogs, he sees, as dogs that will possible harm him, and in turn, placed up an aggressive facade to protect himself against a future possible attack. Fear aggressive dogs like this can become serious threats, especially if their behavior is not corrected sternly and quickly. Training to eliminate this behavior is NOT for anyone who is inexperienced with handling aggressive dogs, so I suggest you get a professional who is experienced in animal behavior. Your dog fears all other dogs, and see's them as possible attackers, who will harm him just like the dog who attacked him did, he needs to learn, that not all dogs are that dog, and that dogs can bring about positive things in his life, but this requires socialization, and few people will risk placing their dog within limits of an aggressive, fear driven dog.You need professional help, or obedience classes, please seek it. Both the Bull Mastiff and the Bulldog are VERY powerful breeds who can cause very horrific destruction if their aggressive/negative habits are not curbed and your dog is a mix.

It is advisable to take care of any your animals