I have a 2 year old male Jack Russell and I surprised my girlfriend with a new puppy, a 8 week old female Cavalier King Charles. Its only been a few days, and it hasn't gone very good, so I'd love to get good advice right off the bat so I can develop a working strategy to make our house happy with BOTH dogs. The JRT was crated from a young pup until I gained total trust in him being home alone and being in bed with us. The new puppy will also be crated. We woke up yesterday, I let the puppy out of her crate, let both dogs outside and after took them both into bed to lay down for a little and get more acquainted with each other and with my girlfriend. My JRT growled twice then snapped bad at the puppy, not biting, but darn close, and too close for my comfort, being that the pup weighs maybe 4 lbs. My JRT is an extremely social dog who loves the park with other dogs(the more the better for him) and has NEVER had any problems with temperament. In other occasions, the puppy has tried playing with the JRT as puppies do(little nipping, etc etc) and the JRT lets it go maybe once, then growls then snaps. The JRT is also much more protective and jealous of my girlfriend and her space hen he is of mine, so he is not a big fan of her showing affection(holding him on couch, etc etc) Any and all advice will be GREATLY appreciated and if I left any details out which would be important in order to give advice please add. Thanks!!

This is all within the realm of natural. Your JRT plays well with other dogs so will probably be fine with the puppy in a week or 2. Snapping without biting is just a typical warning to a newcomer upstart young pup. Let the dog-pup training take it's course while keeping an eye on it. Protectiveness/jealousy is natural too. Pat your JRT and distract him if he starts in.

My JRT still growls at my kitten after 6 months when the kitten gets close to the dog bowl. I use sharp NOs for severe behavior but more drawn out ones for stuff like growling over food. They know. The growl sort of turns into a strangled sound and she looks guilty. Half the time I'm not even around when it happens and the kitten is just torturing the dog anyway, so it's no big deal. Their wolf ancestors engaged in all sorts of hierarchical behavior.

You JRT is just establishing his role as alpha. When he growls it's actually a good sign, he is being vocal, saying I don't like this, the snapping too. If you punish or yell at him for showing these warning signs, he will eventually stop and just bite with no warning. This is how dogs act in the wild. As soon as the JRT is sure of his role then things should even out. It's very important, now for either you or your girlfriend to be the ones in the alpha role to take that stress off the JRT.

Make sure to feed separately, and make sure to reward all the acceptable and good behavior.

The best way to introduce the dogs, would have been on neutral ground, letting the dogs sniff, being mindful for any territorial or aggressive behavior, and slowly bringing the new dog into your home. This is all new and very confusing to your JRT. All he knew was he was the single baby and now he has to share his love and home with a new pup.

Obedience school would be a great way for you and the dogs to bond, creating self esteem in both pups, as well as establishing yourself as alpha. Just remember with all the new excitement of having a new pup in the home, don't forget your JRT.