When a new dogs comes into a household, usually the older dog already in the household is the boss and remains the top dog. However, when dogs read about 18 months of age, they tend to test their place in the pack and since the other one is not that much older, he may feel he has a chance at the top position.
In most cases, it is best to let them try and work it out on their own if no blood is being drawn and there are no serious injuries. However, obedience training is recommended so you can control them without having to touch them as that is when most owners get bit.
The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Additional training can also help if the situation gets worse. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
Most likely they will work things out but you should also learn body language so you can determine what is being transmitted to the other dog by their body language.
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