It is natural for a dog, at one time or another, to get into a fight with another dog. Some dogs are more prone to fighting than others. They either act, or are, more aggressive for a variety of reasons. It is not a good idea to try breaking up a dog fight because you might end up being the one who gets injured.
Dogs act aggressively toward other dogs for a variety of reasons. A puppy who was not properly socialized to other dogs, i.e., allowed to play or romp with various kinds, sizes, and both sexes of dogs, will naturally grow up feeling anxious and tense in the company of other dogs. Some dogs are not social because of their need to defend territory.
The territory might be the yard, the house, or personal territory such as the food bowl or the bed. There are hormonal aspects to aggression toward other dogs. A female in heat, or with puppies, might act aggressively toward other dogs. Two un neutered males will be more aggressive toward each other than two neutered males. A dog who is tied up or restrained in some way acts more aggressively because he senses that he is defenseless. Some dogs act any way they want because they have assumed a leadership position with their caregiver. A dog may act aggressively because he has been attacked by another aggressive dog sometime in his life. A dog may act aggressively because he has learned this behaviour from his mother or he may have inherited this characteristic from one of his parents. He may have been the runt of the litter, learning to fight for a feeding space. As you can see, there are many ways in which a dog acquires a disposition for acting aggressively toward other dogs.
It is important for you not to unintentionally reinforce this kind of behaviour. For example, if your dog growls at another, do not give him reassuring words like, "That ’s okay." Also, do not pick him up. If he growls or snaps at another dog, do not try to physically restrain or soothe him. A dog will sense your uneasiness, and this is a signal to act even more aggressively to protect you! Do not think you are making a good watch dog by saying "Good Dog" when he growls at another dog. You are creating a mean dog.
If your dog is acting aggressively toward other dogs, here are some things you can do.

Obedience training is a first , this will not only give controls over your dog , but also will teach him discipline and self control . Take him for frequent walks where he can be exposed to other dogs. Do not restrain him for wanting to investigate another dog unless you know his intentions. Act and sound happy by laughing or singing, giving your dog the message that you are not concerned by the approach of another dog. Praise your dog for appropriate behaviour, i.e., not growling, snapping, or pulling on the leash. Get control of your dog. Take a leadership position by training your dog in the commands of sit/stay and down/stay. If you have not already done so, get your dog trained . This will help to control his territorial tendencies.