Hello, This is my first post and I hope I'm not being too repetitive with other threads, but I think I have sort-of a unique situation. My Border Collie/English Setter mix is just under 2 years old. I got him when he was a puppy from a rescue. I socialized him well and took him on walks in public areas often to get him used to people and dogs. He came to work with me and played with other dogs and we went to puppy obedience classes as well as dog parks. I'd like to think I did well with introducing him to new dogs but now I'm not so sure about that. A half year ago, he started this aggression thing on a leash. It's not playful and it scares some people. He lunges at other dogs (both dogs on and off leashes) with his fur sticking up and growling. He'll nip at them if he gets close enough. This started slowly, first with just some dogs who I assumed he didn't like (small dogs, usually Toy breeds), now it's almost all dogs he sees while he's on a leash (except for dogs he already made friends with before this all started). He went from a social butterfly to an aggressive dog who I can't take everywhere anymore. He still comes to work with me, but it's become a problem and I have to leash him next to me at my desk. He can't go out for playtime anymore because I never know what he'll do to another dog. Even walking him to and from the car can be a problem if there are other dogs nearby. It seems like a dominance thing when he goes after these dogs, but how do I stop this? Other than this, he is well behaved and I feel like I've tried everything to stop it. I've tried working with him to listen to my commands more so he pays more attention to me, but that doesn't work when he sees another dog he wants to attack. I try distracting him with treats on walks so he doesn't pay attention to other dogs but again, the second he sees a dog he doesn't like he ignores the treats and me. (This is weird because he's very food driven.) Any information or advise is useful, I've tried other suggestions but nothing has worked. I'd love to avoid a muzzle or corrective collar as some suggested because I don't feel that will stop the root of the problem. Thanks for listening!