Remember your very first pup and the plans you had for both of you? Your pup will have outstanding manners. Nice to people and other dogs. Come when you call. No problems whatsoever, just enjoying doggy life. It is a nice dream isn't it.
Somewhere down the road we find out that even if we have the nicest and most obedient of pups, we still need two to tango. Not everybody we meet is as nice and well-behaved as we would like them to be. Even dogs that seem to have great personalities, might just not get along. Like people.
Owning a dog means you have to be prepared for a dog fight. Unfortunately but inevitable. Even if your pup is not reactive, you will meet one that is.
You will be too late
Don't fool yourself you are able to distinguish a dog fight from a skirmish. Dogs just read each other quicker, and they decide quicker. You don't have a chance. When you are still contemplating about if what you are seeing is a dog fight or an impolite exchange of macho dog talk, things are already enfolding. Acting on a fight which is not is just as dangerous. Shouting and intervening will only add the stress that can trigger an actual fight.
Avoid
Ninety percent of the solution is to avoid. Tune in to the body language of the approaching dog and your own dog beforehand. Is your dog avoiding the dog that is coming, or showing stiffness? Be alert. With stiffness, walk in the other direction immediately. When your pup starts to move in a different direction and sniffing casually, follow them and don't force them to meet or walk by the other dog. Support them in their actions and walk past in a circle. Handlers of search-and-rescue dogs have a great principle to live by: "Trust your dog". This is one of those times. Giving your dog this trust will strengthen your teamwork and your dog's confidence.
If possible, avoid places where dogs can be off-leash, like dog parks, when you don't know all the other dogs and dog owners. Only have an off-leash walk or dog park visit with people and dogs you know and can trust.
Approaching dog
When you are good at avoiding you could still walk into an off-leash dog or a stray that might turn aggressive. That's why you should always carry a citronella spray with you. Or at least a water-bottle, even better pepper-spray when that is legal in your state/country. When you are unsure about the dog's intentions try shouting to make him think twice about coming any closer. Use the spray if that doesn't help. Don't wait until it is too late. Once there is a fight, the spray will have little effect. Think about before hand on what exact distance you would like to use both, this will make you act quicker in the situation itself, when there is hardly time to think. When nothing helps to stop the dog from approaching drop the leash of your dog. This might seem strange, but dropping the leash removes tension from your dog and might enable them to send the correct message to the unwanted visitor. On her blog, Laurie Luck tells how it helped her to avoid a dog fight.
In a fight
I am sure instinct will take over now, despite of what I will write. But please stay with me on this one. When you are in a fight, drop the leash of your dog to give him a better chance to defend himself if you didn't already.
The last thing you should do is trying to step in and grab either dog by its collar, spark or kick. The chances for a dog bite are high and you can get severely injured. If the other owner is around you should both try to grab a dog by its hind legs or tail and trying to pull them away from each other. Usually there is no time to explain, just start pulling on their dog so they get the picture. Although this is also with risk, and can also provoke a bite. On her blog, Karen Friesecke wrote some other options down that might also work.
Everybody is different, and I do respect when you are that kind of fearless person that in a split-second jumps in the middle of a fight to protect your dog. But beware the consequences. My dad is such a person, and he has been in a dog-fight three times during the last year in which he got bitten each time. The last time it was that bad that they had to do surgery on his hand twice, and we feared he would loose the function of his hand. Do not underestimate the force of a dog bite. And when you get severely injured, what does that mean for the life of your dog?