Instructions
Use the fact that dogs are territorial to anticipate problems. While the degree to which a dog will defend its territory can vary depending on the breed of dog and how well it has been socialized, most dogs will view any new dog in its territory as a potential treat and behave accordingly.
Introduce a new dog in a neutral location like a park rather than your older dog's backyard. This will help to reduce the chance that your older dog will feel the need to attack the "invader." While you may believe, and want, your older dog will be "pack leader," it doesn't always work out this way. You can communicate this idea to your dogs by feeding the "top dog" first and placing its bed in a prime location, like at the foot of your bed.
Remove all food when you are introducing new dogs to each other, since nothing is more likely to breed fear and resentment in the old dog than thinking that the new one is taking its food.
Encourage them to sniff each other while you remain in control. Control can be achieved by having both animals on a leash. If you are really concerned about the chances that a fight might break out, you can place a fence or baby gate between the dogs so they can sniff each other safely.
Look for physical clues. A dog will become rigid and stiff when it is feeling anxious. Puppies and younger dogs will normally behave submissively toward an older dog by licking at the mouth of the older dog and groveling with its tail between its legs, sometimes crouching or falling down on the ground.
How to Prevent a Dog Fight
The last thing you want to happen when you bring a new dog home or when you are taking Fido to play at the dog park is to see your dog involved in a dog fight. Once dogs begin fighting, it can be difficult and, indeed, dangerous to intervene in an effort to break up the fight. The best approach is to prevent fighting to begin with.