One of the first steps is to try to identify what the underlying cause is. It sounds like you have tried but haven't been successful identifying a starting cause. That's ok. We can still look for way to remedy the problem. One things to consider though -- is one cat has become subtly more territorially aggressive and causing fear aggression in cat number two?? If this is the case, it helps to start with a clean slate. Almost pretend these are two new cats that do not know each other and start forming a new relationship. We may never know what started it but we can work toward a new relationship. Here are some ideas shared by Suzanne Hetts, PhD, at the Tufts Animal Expo on the topic of Helping Cats Co-Exist. Structure the environment to decrease competition among the cats. No cat should have to face harassment and threats from another cat while attempting to meet his basic physical needs. There should be multiple locations, or stations, for all the important things in life. Multiple feeding stations should be provided, so that the cats don't have to jockey for position at one food
bowl. Lining up several food bowls right next to each other isn't sufficient-food bowls should be at several different spatial locations, depending on the number of cats and the degree of conflict between them. Similarly, multiple litterboxes should be provided, in numbers at least equal to the number of cats, perhaps even a one or two more. These boxes should be in different rooms, and even different floors in a multi-level house. Multiple objects for scratching are also important. Each cat may have individual preferences as to the location and texture of the object he or she likes to scratch, and these factors should also be taken into consideration. Scratching objects need to be easily accessible, and in areas where cats prefer to scratch. Locating one in the corner of the basement is probably not going to be helpful. Provide multiple cat perches, which allow the cats to use the vertical space to their advantage. Multiple resting places at different heights in various locations should be provided, in numbers relevant to the number of cats in the household. Cats also need hiding places. Some are provided naturally, such as under the bed, but some rooms may lack them. Putting an upside down cardboard box with one side cut away behind the couch, a small decorative cat screen across the corner of a room, are examples of ways to create hiding places in rooms which have few, or none. Another idea is redirected aggression. Cats are prone to problems stemming from redirected aggression. Redirected aggression occurs when the aggressive response is directed toward a target that did not initially trigger the response. For example, the resident cat may become agitated by the odor of the newer cat or once Grimm was introduced back into the house. Because cats can stay aroused for several hours, the release of the aggressive behavior may occur hours after the cat's encounter with the initial trigger. This can make redirected behavior very difficult to categorize as such. Cats are also at risk for problems stemming from redirected aggression when they view outdoor cats through a window or door. Other examples of common triggers for a redirected response are: the odor of another cat on a person entering the house, when an exclusively indoor cat gets outside and becomes aroused and anxious, high pitched noises, when a cat is harassed by a dog, and when a person attempts to intervene in a cat fight.Another idea:I recommend actively work on making the appearance of the other cat associated with rewarding activities e.g. feeding, game play, patting etc depending on the cats' preferences. Doing some sessions where they are a long way apart from each other and, for instance, offering part of their daily ration or some extra special treats, contingent on the appearance of the other cat, can bring about a change in attitude toward one another. The use of a harness or see through barriers can reduce the risk of fighting. Medication may be warranted for one or both cats. You might prefer to medicate one in the first instance, if you are unsure if one is triggering the other. You can always add medication for the second if results aren't adequate. Prozac or buspirone for the most likely aggressor would be a reasonable first option. I'd also look out for any new cats in the area or any other wildlife that may have triggered the first fight, with a view to controlling access to these if at all possible.Have you ever heard of boarmate? The concept of using this steroid hormone is to "dress" the cat (on which you applied it) as a masculine male that others will not disturb -- ie it would be used on the more submissive cat. Hence, when the male, neutered or not, will approach the other cat he will avoid the fight. Personally, I have never used it. I am somewhat skeptical using things like that, and have not seen any good piece of evidence it actually works -- but have heard from other vets and clients that it worked wonders with their cat. I don't think it can hurt trying. It should not cause any adverse effects. Another benign option is Feliway diffusers.Unfortunately the prognosis for intercat aggression is somewhat poor due to the time and effort needed to correct the behaviors. There was no way for you to predict what would or could happen with the moves and that can be difficult too because no 2 cats or cases are alike. Other options include fluoxetine for the bully cat if he is the bully and the smaller cat is more of the victim, separation and slow re-introduction as if new cats to the environment. The separation helps deter the risk of them hurting each other. It also helps so that you can be there for each re-introduction so all the interactions can be documents. Sometimes it appears that there is dominance aggression but it turns out it is actually fear aggression. Fear aggression is handled slightly differently by rebuilding each cat's confidence independently of the other cat. He may now defend himself pre-emptively from the other cat. Most cases that I have seen where someone wants to call it territorial aggression actually have a fear component and may even just be fear aggression. The hissing can be misinterpreted by each cat. For example, one hisses and it seems like aggression due to dominance but it is actually fear. The other responds by hissing -- he is afraid too, but his hissing doesn't help --it reinforces to the male that there is something to be concerned about.