We adopted a new 3 year old female shih-tzu named Lexi from the shelter yesterday. It is clear that the dog was not abused at her last home, and she is pretty well trained (she sits/stays etc. on command). We did realize she was territorial because she nipped me a couple times when I tried to pet her while she was chewing on her rawhide, but other than that she's been a total sweetheart with us. Anyway, when we brought her home yesterday, My old dog Charlie (a 2 year old norfolk terrier) was home. He is the sweetest dog in the world! He is super submissive and gets along with every dog he's ever met, couldn't harm a fly. Lexi and Charlie were, for the most part, fine with each other. When he would go up to her she would growl, and show her teeth, but as long as charlie left her alone and ignored her they were fine. Today, my son took charlie out for the day just so that Lexi could adjust, get comfortable, and have less things to be stressing about. Unfortunately, when we brought Charlie home Lexi was a completely new dog with him. She was lunging out and attacking him. We tried everything! We put them on leashes and she lunged and bit him. We introduced them through a screen door... she attacked and almost broke the screen! Now Charlie is absolutely TERRIFIED of her. He won't walk into the same room as her, even if she is in her kennel or being held. Charlie would do anything for me and if I wanted him to he would let me bring him near her again, but it seems cruel to use his trust and put him in a situation where he might again be snapped at. What should I do? I don't want to have to bring Lexi back to the shelter, but if she is this aggressively territorial after less than 2 full days, I can't imagine how she will be after months. It is clear that she is a dog that would be perfect for an elderly person who she can be entirely loyal too, won't be exposed to strangers or other dogs, and can just live in her own comfort zone, but I have made this commitment to give her the amazing, loving and happy home that I have for my children, Charlie, and my dogs before him. I need to find a solution to her aggressive territorial behavior because I can't have her threatening the stability, security, and overall well-being of my dog, family, and friends, it would be unfair to them. I am determined to do whatever I can to make this work for her, and I really hope I can find a solution because it would break my heart to have to bring her back and once again put her back into shelter life. Please please help! Any suggestions would be greatly appreciated!

A Shih-tzu isn't a breed who will be a 'shelter dog' for more than a few days, so I would bring her back. I suspect that the amount of training required to keep her from nipping people and attacking your other dog might not be what you will want to go through. She will find a home with no other dogs, easily, as long as the shelter knows that's what she needs. You have children and don't need to take the risk of her biting them either.