However, many aggressive problems between cats can be successfully resolved. To do this, you may need help, both from your veterinarian and from an animal-behavior specialist who is knowledgeable in cat behavior. Cats with aggression problems may never be best friends, but can often learn to mutually tolerate each other with a minimum of conflict. Working with aggression problems between family cats will take time and commitment from you. Don't give up without consulting the appropriate experts.
Cats are very territorial, much more so than dogs.
Territorial aggression occurs when a cat feels that his territory has been invaded by an intruder. Depending on where your cat spends his time, he may view your whole neighborhood as his territory. Female cats can be just as territorial as males. The behavior patterns in this type of aggression include chasing and ambushing the intruder, as well as hissing and swatting when contact occurs. Territorial problems often occur when a new cat is brought into a household, when a young kitten reaches maturity or when a cat encounters neighborhood cats outside.