Dogs can be aggressive with other dogs, especially if they have not been properly socialized with other dogs in puppy-hood. Sometimes a dog that is naturally dominant has trouble with other dogs especially in puberty. Sometimes a dog has a specific experience (e.g. a dogfight with another aggressive dog) that causes it to become aggressive toward other dogs in general as well. Whatever the reason, it is well worth your time working on your dog's aggression toward other dogs. You will probably get the best results, especially with a problem dog -- extreme aggression, for example -- if you contact a local trainer (preferably one that specializes in problem dogs) for individual help. However, there are some common-sense things you can do.
First a bit of basic dog psychology: friendly behaviors include moving side by side, sniffing butts, tails wagging at body level (not up high or over the back). Not-friendly behaviors include meeting face-to-face, esp. a face-to-face approach, ears forward and tail over back.
Force them into friendly behaviors as follows: walk the dogs in parallel on leash. They should be close enough to see each other but not close enough to snap at or touch each other. Be careful when you two turn that the dogs don't tangle. Make sure one doesn't get ahead of the other: keep them parallel. Keep this up until they relax. Slowly start walking closer together as behavior permits.
Hold one dog on leash in a sit. Have food treats and a water bottle handy. Walk the other dog toward it, to about six feet, then turn away (increase the distance if the sitting dog snarls). The idea is to turn away *before* the sitting dog shows any aggression. If the dog shows no aggression, reward it with a food tidbit or verbal praise. Do NOT touch the dog (stand on the leash or tie it down). If it does growl, spray it with water. Switch the dogs so that each experiences sitting or walking toward. They are learning that good things happen without defensive behavior. As they improve, start walking a bit closer before turning. If the sitting dog snarls, do NOT turn the other dog away: the person with the sitting dog should correct it and when the dog subsides, THEN the moving dog should turn away.
Finally, holding the head of one dog, but allowing it to stand, have the other dog investigate its rear briefly. This is really the extreme extension of the above.
These exercises have several purposes. One is to force the dogs to consider themselves friendly by engaging in the behavior of friendly dogs. The other is to teach both dogs that an approaching dog is not necessarily grounds for aggression.
This will take a lot of work, probably over a couple of months, but they will work, and what's more, should reduce tensions with other dogs as well (i.e., not only between the two specific dogs in the exercises).
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I have always been told by Vets that if the dogs are fixed this will help with agression, roaming, e.t.c. because it reduces their hormones.
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Help!!!! How Do I Stop Dog Aggression Against Other Dogs 
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