We have a 2 yr old mixed breed (about 80 lbs) that has always been friendly with other dogs including three dogs in the neighborhood. We got a puppy last weekend and he has been very protective of her when she has been around our neighbor's dogs. He has been fine with the other dogs then when the puppy showed up he basically attacked them. How can I curb this behavior? The other dogs weren't trying to hurt the puppy or even behaved aggressively towards her.