If you want to understand how to stop dog aggression let me start by asking you two rhetorical questions!
Firstly do YOU start the aggression with your dog simply joining in?
Secondly does your dog listen to you just before it behaves the way it does when you try show it another way to behave?
The answer will of course be the following:
At the point your dog starts to become aggressive he is taking no notice at all of you.
He is making his own decisions and will not listen to you if you try to show him a different way to behave. What he is doing is simply too important to him and is the right thing to do. Dog aggression is nearly always done in order to protect, their pack and their own lives.
Firstly there is of course a whole range of different types of aggression from dominant to fearful and everything in between. Then there is aggression that occurs the whole time and other aggression, which is very erratic, and random depending on a number of differing factors. We could also look at what your dog is aggressive towards; it could be people, animals, other dogs or objects.
The way to stop dog aggression however is very much the same, or at least the cause of the problem is the same. Your dog thinks that it is the pack leader, becomes fearful and attacks to protect, you and himself. Dominant dogs will be more proactive, often attacking when they still have the option of running away, fearful dogs will only attack if they have no place to run. All the other factors pale into insignificance compared to this.
The most important concept to grasp if you want to understand how to stop dog aggression is that your dog must first look to you as the pack leader in the home. (This is the easiest place to convince him you are the decision maker.) Only then can you convince him that you are the pack leader on the walk. There are some fantastic video sites now that show you exactly how to become the pack leader.
Once you have convinced your dog that you are the pack leader outside then upon reaching the point where he usually is aggressive you will find that he will actually start to take notice of how you are behaving! If you aren’t then your dog will probably continue to ignore what you are doing at this point forever.
Just remember, dogs are pack animals and they follow the pack leader.
One of the best examples of a professional dog trainer putting this all into practice is The Online Dog Trainer. The site has live videos of this method being demonstrated and explains exactly how to stop dog aggression by simply convincing your dog that you are the pack leader.