We adopted our pit-mix Jimmy from the shelter when he was 6 months old, we have had him for almost a year now and he has always been extremely sweet. He is exuberant and playful and likes to jump on visitors (which we have tried to consistently correct) but he was never aggressive or territorial. He was just very playful. Well two weeks ago when we were home the mail man drove up to drop of a package, and this time our dog was aggressive. His fur was standing up on his neck and he snapped at the mail man as he was walking away. Ever since then he has shown the same aggressive and territorial behavior with people walking down our road and just today, the garbage collectors. We have him contained in a wireless static fence system but it is not fail proof and doesn't help if people come into our yard. In fact, about 6 weeks ago the batteries in his wireless collar had died and he got into the road approach a lady walking her German Shepherd (the German had been in our yard before, because they roam the neighborhood) but the German was not interested in Jimmy this time and they got in a scuffle by the time I could pull Jimmy away. We have resorted to putting him on a chain when we are not home, which I hate and know just adds to his confusion. I feel defeated and scared and worried. Does anyone have any advice?