Inter-aggression among cats is a big problem for people who have more than one cat. The different kinds of inter-aggression could be related to territory, fear, dominance or it could be redirected aggression.
Cats are territorial animals. They mark their territory through urine and feces and other scent glands. By doing this, conflict between cats is reduced. Cats in a house tend to live by certain territorial rules. It is only when these rules are broken that territorial inter-aggression occurs. This is most common when a new cat is brought into the house. The best thing to do in a case like this is to introduce the new cat to the older cat gradually. This could take some time. If the new cat is accepted by the older cat right from the start, with very little aggression, then you could do away with the gradual introduction.
Territorial aggression between cats in the same household could start slowly. The stronger cat could become more possessive about its space and threaten the weaker cat. If the cats are of equal strength, you could face problems such as spraying and other markings.
Another form of inter-aggression could be due to non recognition. For instance, you have two cats that get along relatively well together. You take one cat to the vet. You bring it back and it smells different or behaves differently. The cat that was left behind does not recognize the cat thats been brought back. They fight and this leads to territorial aggression. In situations like this, it is best to keep the two cats apart till they get used to each other again.
Redirected aggression is quite common. This happens when aggression meant for an outsider is turned onto the other house cat. For example, two cats are sitting together quite peacefully. Suddenly they hear a loud noise and both assume defensive postures. They look at each other and think that one is going to attack the other. A fight starts with both cats trying to defend themselves. Redirected aggression can be dealt with by separating the two cats for some time. Give them enough time to calm down and try to reintroduce them to each other at meal time.
To prevent inter-aggression among house cats, you could take the following precautions.
Get all the cats in your house neutered. Always keep their nails trimmed. This could prevent injuries during fights. Use toys to play with your cats everyday for a while. This reduces anxiety and helps to deal with excessive energy in cats. Another useful trick is to use the towel of one cat while rubbing down another cat. In this way, the cats can get used to each others scents.
Inter-aggression between cats can get quite bad. Never reach between the cats to stop a fight. Use a blanket or anything else that is handy to break up the fight. Keep the cats apart and give them enough time to cool down. Reintroduce them only after they are completely calm.