I have two 8-year-old neutered brothers who have lived their whole life on 20 acres with indoor/outdoor freedom and virtually no other cats around. They get along fine with the occasional squabble that gets slightly out-of-hand. I recently had to move them into “town” where I have a house on an averaged-size lot with many cats in the neighborhood, including my tenant in the back who has a grown neutered male cat (Marley). I just started letting my cats go outside recently. Today my “bolder” cat Mac encountered Marley. Marley is pretty mellow (and huge) and seemed only interested in introducing himself, but Mac was instantly hostile (hissing/growling). Marley eventually wandered away and I put Mac back in the house. The next time I let Mac out I heard the growling/hissing again and was there just in time to see Mac attack Marley. They really went at it with neither of them backing down. It took 4 times of me separating them and watching them immediately attack each other again before I was able to get them far enough away from each other to get Mac in the house. Is there anything I can do, or any hope, that this will not be a constant issue?