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. It's not uncommon for a cat to be territorially aggressive toward one cat in a family, and friendly and tolerant to another. Adult male cats normally tend to threaten, and sometimes fight with, other males. These behaviors can occur as sexual challenges over a female, or to achieve a relatively high position in the cats' loosely organized social-dominance hierarchy.