Aggression between dogs can result in injury to dogs and/or to the people trying to separate them. The behavior can consist of growling, snarling, barking, lunging, snapping and biting.

Aggression between unfamiliar dogs can be due to dominance, fear, or protective behavior over territory or owner.

This aggression can be elicited by dominant gestures or postures from either dog. These can include placing head, or feet on the back of the other dog, dominant body postures such as eye contact, high tail and stiff legged approach. Owners may inadvertently reinforce the behavior by leash tightening and vocal cues. These may signal to the dog that the impending approach is problematic. Unfortunately leash restriction does not allow the dog to react with a complete rate and range of responses including body postures, approach and withdrawal. Additionally, this behavior can be elicited by smaller dogs that repeatedly "attack" larger dogs who are attempting to avoid interaction. Dogs of near equal dominance and those of the same sex are most likely to compete for dominance. Some extremely bold or assertive dogs will fight rather than back down when challenged. Although dominance challenges may be a source of aggression when two dogs are meeting each other for the first time, most dominance hierarchies are established with posturing and no fights. It is likely therefore that fear, territorial behavior and learned components, would contribute to an attack. Dominant aggressive dogs may be over-assertive and/or overprotective if the owners do not have good control or have taken a subordinate position in relationship to the dog.

This aggression is primarily exhibited when unfamiliar dogs are on the resident dog’s property, or what the aggressor considers his territory. Some dogs get highly aroused at the sight of other dogs on their territory and may jump fences, or go through windows or doors to get to the intruder.

This aggression is very common in aggressive encounters with other dogs. The diagnosis is made based on the body postures and reaction of the dog when faced with another dog. The fearful dog will often have the tail tucked, ears back and may lean against the owner or attempt to get behind them. They may be barking at the approaching dog and backing up at the same time. Often the dog is avoiding eye contact. This behavior can be precipitated by previous aggressive attacks from which the dog could not escape and sustained injury. Owners that try and calm their aggressive dog may serve to reinforce the aggression, while those that try and punish the dog will only serve to heighten the dog’s fear and anxiety in relationship to the stimulus. Good control can help to calm the dog, while owners who have their dogs restrained on a leash (especially with a choke or pinch collar) and have poor control often have highly defensive dogs. Dogs that are restrained on a leash or tied up are more likely to display aggression when frightened, because they cannot escape.

Learning and conditioning aggravate most forms of interdog aggression. Should threats or aggression result in the retreat (or removal by the owner) of the other dog, the behavior has been successful. If the owner tries to calm the aggressive dog or distract it with food treats, this may only serve to reward the aggressive behavior. One of the most common mistakes is to punish the dog that is aggressive toward other dogs. This usually serves to heighten the dog’s arousal, and teaches the dog that the stimulus (other dog) is indeed associated with unpleasant consequences. Many owners, in an attempt to gain more control, then increase the level or type of punishment (e.g. prong collars) which further heighten the dog’s arousal and in some cases may lead to retaliation and defensive aggression toward the owners. And, if the dog to dog interaction results in pain or injury to one or both dogs, the dogs will quickly learn to become more fearful and aggressive at future meetings. In short, if the owners cannot successfully control the dog and resolve the situation without heightening the dog’s anxiety or increasing its fear, the problem will progress with each subsequent exposure.

Prevention starts with puppy training and socialization. Early and frequent association with other dogs will enable your pet to learn proper interactions and reactions to other dogs. This can be very helpful in prevention of aggression to other dogs.

You must have good control of your dog. This means that your dog will take contextual cues from you, and may be calmer and less anxious in the presence of new stimuli. Moreover, the dog should reliably respond to commands to sit, stay and quiet. If necessary, the dog may need a head halter to give you additional control. When in situations where the dog may encounter other dogs, a leash is necessary.

For territorial behaviors, what is most important is to prevent the dog from engaging in prolonged and out of control aggressive displays both in the home and yard. Aggressive displays include barking, lunging, fence running, jumping on doors, windows and fences. These types of behaviors should be discouraged and prevented. One important component is teaching your dog a "quiet" command for barking.

First and foremost, you must have complete control over your pet. This not only serves to calm the dog and reduce its anxiety, but also allows you to successfully deal with each encounter with other dogs. Leashes are essential and the use of head collars and/or muzzles is strongly recommended for dogs that will be in situations with multiple dogs.

Begin by establishing reliable responses to basic obedience commands. If the dog cannot be taught to sit, stay, come and heel, in the absence of potential problems, then there is no chance that the dog will respond obediently in problematic situations. Reward selection can be critical in these cases, since the dog needs to be taught that obedient behavior in the presence of the stimulus (other dog) can earn the dog favored rewards. The goal is that the dog learn to associate the approach of other dogs with rewards.

Long term treatment consists of desensitization (gradual exposure) and counter-conditioning the dog to accept the approach and greeting of other dogs with obedience and rewards. This must be done slowly, beginning with situations where the dog can be successfully controlled and rewarded and very slowly progressing to more difficult encounters and environments. The first step is to perform training for its favored rewards, in a situation where there are no dogs present and the owner is guaranteed success. Food or toy prompts can be used at first, but soon the rewards should be hidden and the dog rewarded intermittently. The selection of favored food or toys is essential since the goal is that the dog will learn that receiving these favored rewards is contingent on meeting other dogs.

Once the dog responds quickly and is receiving rewards on an intermittent basis, training should progress to low level exposure to other dogs. If the owner’s training and the rewards are not sufficient to control the dog in the absence of the other dogs, then utilizing a leash and head collar, selection of more motivating rewards, and seeking the assistance and guidance of a behaviorist should be considered. The next steps in desensitization and counter conditioning rely on a stimulus gradient. In other words your dog needs to be controlled, (preferably with leash and head halter) and respond to commands and rewards in the presence of gradually more intense stimuli.

Begin with a calm, and well-controlled second dog, in an environment where your dog is least anxious or threatened, and at a sufficient distance to get your dog to respond to your commands. Gradually the dog is exposed to dogs at closer distances and in more familiar locations. Using the head halter and a prompt (reward prompt, set of keys) it should be possible to keep the dog focused on the owner and sufficiently distracted. While dogs with fear aggression may improve dramatically, dogs with dominance-related aggression that are trained in this manner usually do not greet other dogs, but should walk calmly with their owners and not initiate fighting behavior.

Dogs that are exhibiting territorial aggression should be retrained in much the same manner, but the gradient of stimuli will need to be adjusted. Begin in the front hall or on the front porch with no other dogs around. Then with the dog controlled in the hall or on the porch, other dogs could be brought to the perimeter of the property. Over subsequent training sessions, the dogs could be brought closer to your dog, or your dog could be moved closer to the other dog.

Another way to disrupt the undesirable response and get the dog’s attention is to use an air horn or shake can. Once the inappropriate behavior ceases, and you get your dog’s attention, the dog should be redirected to an appropriate behavior such as play. The greeting should be repeated, until no threats or aggression are observed.

Success can be achieved in a number of ways, but head halters are generally the most important tool. Head halters provide enough physical control that the desired behavior can be achieved (sit, heel) since pulling up and forward, turns the head toward the owner and causes the dog to retreat into a sit position. With the dog’s head oriented toward the owner and away from the other dog, lunging and aggression can be prevented, and the dog will usually settle down enough to see and respond to the prompt. Rewards can and should be given immediately for a proper response (sitting, heeling), by releasing tension on the leash. If the dog remains under control with the leash slack, the reward (toy, food, affection) should be given, but if the problem behavior recurs, the leash should be pulled and then released as many times as is necessary to get and maintain the desired response. The dog’s anxiety quickly diminishes as it learns that the other dog is not to be feared, that there is no opportunity to escape, that its responses will not chase away the other dog, that responding to the owner’s commands will achieve rewards, and that the owner has sufficient control to achieve the desired behavior (which further calms the dog). Also since there is no punishment or discomfort that might further aggravate the situation and rewards are not being given until the desired behavior appears, fear and anxiety will be further reduced.