Dog-on-dog aggression can stem from fear, resource guarding, barrier frustration or even proximity sensitivity and leash reactivity. I’ve read that management is key. Consulting with a professional trainer or vet about desensitizing and counter-conditioning techniques. Also, understanding your dog’s personal reactive threshold level during training is critical. Once your dog has maximized his/her reactive threshold, it’s almost impossible to get him/her to redirect his/her focus. Here’s a link you may want to check out.