Originally Posted by shengmei
You can also borrow a cat trap from the local humane society, trap the white cat and then bring the white cat into the animal shelter. Then Thomas can finally get some peace and quiet.
There are so many sweet, innocent, homeless cats in the world. The world has no place for angry, aggressive, and belligerent cats.
Trapping: Taking such a step escalates any potential situation to just plain confontational right at the start. Showing goodwill first gives a person better footing for later actions. Before this step is attempted, some earlier ones should be taken first, such as IDing the owner, finding out where the cat belongs, and discussing the situation with them.
We can only guess what the other cat(s) are like, and like Spacemonkey said, it's unfair to impose human characteristics on an animal. Looking at it in cat terms, they've been roaming their 'territory' before Thomas came, and *he's* the intruder to them, regardless of whose human back yard. It means the the cat will be 'aggressive' and a 'bully' in defending his neighborhood, but not in human terms.
Back OT: after typing the above, it occurred to me that one way for the cats to know there's someone new in the 'neighborhood' is whenever you have time, consistently shoo off other cats that you see in the back yard, with Thomas nearby. It won't solve the problem, but it may help a little.