My dog is a mix - mostly lab and border collie, but some other stuff thrown in for good measure. I suspect that she has a good dose of pit bull in her. We live on ten acres and she has free run of it. She is good about coming when called, does fifteen tricks, doesn't beg at the table etc etc etc She's quite smart. But, she doesn't have much contact with other dogs. And now she seems to be extremely dog aggressive. I have friends who would like to come visit with their dogs and it always turns into a big mess. Athena refuses to back down even when the other dog is 100 pounds bigger than her. My dog is usually the aggressor and she is usually the one who won't let it go after the initial skirmish. What can I do?? Other people don't want to risk their dogs by letting them duke it out. Please help!