I have a two year old male working line GSD. I also have two cats (one indoor and one outdoor). I had the indoor cat when I brought home my dog at ten weeks and socialized them so he never chased that cat. At nine months, my dog started started showing signs of dog reactivity (he use to react to small dogs running at the end of their leash) so I began working with a trainer who allowed us the opportunity to expose my dog to calm neutral dogs. He did well and no longer showed signs of dog reactivity. Around that time, I got an outdoor cat for my garden area. I tried to socialize them but my dog acted aggressively toward the cat so I asked my trainer to help me with this. We worked on sessions of impulse control with his cat but in the end, after my dog attacked his cat, he told me I would always need to manage my dog around cats or he could kill them. Since I live on a three acre lot I have lotsa squirrel, deer, rabbits etc so I have worked on a strong recall (we began obedience work early with a trainer and worked through AKC group classes through Competition Novice by nine months) My dog will also down on command while in a full out run. He no longer chases squirrels or bolts after deer but he will tree the cat every time. (Photos attached are from last year. I also have a video of my dog attacking the trainers cat. Very disturbing! He went from a down to a powerful sprint in seconds.) If there’s something I can do and or didn’t do right, I’d appreciate your helpful input on How to Stop my Dog From Chasing Outside Cats!