A while back I was asked about how you get a cat to stop attacking another cat. While each individual case is different there is a basic form that is used in these reactive situations. You have two goals: Desensitization and Differential Reinforcement.
Differential Reinforcement is when you ask for a new incompatible behavior to replace the old behavior. The old behavior would be attacking the other cat. The new behavior would be going in the opposite direction of the other cat. To start we train a "target" behavior. This is when an animal touches a body part to something. All of my dog clients are taught to touch their nose to the human's hand. With cat aggression situations I prefer to use a wand with a ball on the end. This is because cats go in more places and the use of the tool creates a safety barrier between the cat and your hand if they become upset with the situation.
Once the target behavior is taught you now have a way to ask the cat to go in different directions. At this point, we add the stimulus that is causing the attacking behavior, the other cat. For safety, we begin in a "howdying" situation. "Howdying" is a term used in the zoological field when you introduce animals together with a barrier in between. They can see, smell, and yell at each other but they can't make contact. After they safely see each other we will ask the cat to target so that they start looking away and/or moving away from the other animal. If they choose to look away or walk away without being asked, then they are marked and rewarded for that behavior. The end behavior is to avoid the other animal.
All of this will naturally begin the desensitizing process. To desensitize we simply get the cat used to the stimulus that previously caused agitation. When the cat sees the other animal there should always be a mark and a reward to start creating happy memories with that other animal. This will reduce the strength of the reactions over time.
Desensitizing and Differential Reinforcement work together as a feedback loop continually helping the other process.