Some cats like humans just don't want to know. Probably like this because your cat came to the others territory and the matter has never been resolved by the cats. Don't worry cats only fight as a last resort most of it is hissing and show. I understand you protecting your cat butit may be an idea to let nature do it for you.
That is only a suggestion but please remeber injury is the last thing any cat will risk.
Hope this helps even a little?