cats strange behavior?

Question
I have three cats, and recently got a new kitten. The older three are all around the same ago, 2, and grew up together, although the black cat is not of the same litter as the two grey tabbies. They never had problem with each other before, and are all fixed.
A while ago a new neighbor moved into the house who's yard is adjacent to ours, and he has two or three cats, too. His cats are outside cats, while ours are mostly indoors. They go into the backyard sometimes, supervised. The nieghbor's black cat sometimes comes into our yard and crosses our side lawn to get across the street to the woods. Our cats can see it, and the male grey tabby freaks out, meowing and running back and forth between the windows. He is normally a very mellow cat.
The other two cats have accepted the new kitten quickly in the week she's been here, but I think the stress of the new cats is taking a toll on Pierre, they grey tabby. He hasn't lashed out at the kitten, usually avoiding her, running away when she comes near, and he hisses and growls, but not as much as the first few days. But over the past few days, he's been picking fights with our black cat Shadow. Shadow doesn't want to fight, but Pierre hisses at him and starts scratching. Shadow usually runs away, and we break up the fight by clapping or starting Pierre.
Is he just stressed out and lashing out at Shadow? He's shown no agression to his sister or even the new kitten. I don't know the sex or condition of the neighbor's cats, but the black one is the only one that leaves their yard. Could it be possible for him to confuse the two black cats, even though ours is familiar and much bigger than the other one?

Answer
Sabrina,

No, Pierre is not confusing the black cats.  In all probability, Pierre is the alpha cat in your household.  The advent of the new kitten and the strange cat from next door wandering over are brining out those territorial instincts in Pierre.

What Pierre is doing is not unusual.  The best thing, I feel is for your cats to redefine all their inter-relationships under the new circumstances.  To do this, you need to leave them be to sort it all out.  Pierre will not really hurt Shadow, but they do have to come to terms and that will not happen if there is any human interference.  I have two rules when it comes to cat relationships: (1) No human interference and (2) Humans do not understand how cats relate to one another.  Whatever the outcome of the new relationships, you will just have to accept it as there is not way of changing them!!!!!

In truth, I doubt that Pierre and Shadow are really fighting.  Cats can play incredibly roughly (they have ultra tough skin).  We tend to attach human motives and emotions to their games, but we do not really understand.  If you saw how roughly little kittens play with each other, you would not be very concerned about Pierre and Shadow.  Things may never go back tot he way they were, and I would just leave them alone and see how it all, eventually, does work out.

Please keep me posted.

Best regards... Norm.