Aggressive Cat behaviour

Question
I have two male cats age 10, both litter mates. Both are neutered and declawed. They have lived in virtually harmony for most their exsistance.  In the last two days, one cat, Max has begun to attack the other cat Carlo without provocation. Carlo starts to growl from deep in his belly the minute Max comes near him.  If I don't intervene a fight insues with screeching I have rarely heard.  I kept them separted last night.  I let Max out briefly so he could void is necessary within seconds he went after Carlo.  What am I to do?  I am at a loss having never dealt with this before.  Thank you for your guidance.  By the way, nothing in their enviroment has changed to trigger this behaviour. Best Regards, Kim

Answer
Hi Kim!  I can't guarentee that this will be helpful, but I have seen this behavior in the past caused by two things.  
Often times, when two or more animals are living together and are close, they will turn on eachother when one is sick.  In this case, I would say Carlo may be the one that is either sick, or has a sickness "brewing."  I know it may sound odd, but animals can sense illness.  In the wild, before cats were domesticated, if another cat or kitten that they traveled with or homed with had gotten ill, the others in that group would kill it, as to not hold back their travels, and to decrease suffering in that animal. Max definitely will not kill Carlo, so don't worry about that, but in this case, I would say, that if this is the cause, Carlo has a bit of the feline instinct in him.  I would definitely have Carlo and Max checked by a veterinarian to ensure that both cats are healthy.  Kidney disease is unfortunately, very common in cats, and often times no symptoms will show until the disease has progressed further.  Hyperthyroidism and diabetis are also very common.  The reason I mention these ailments, is because they are extremely common in older cats, and being that your cats are 10, some of these ailments may begin to take form.  
Another cause would be if one of your cats has been to the vet recently.  The scent of the vet's office is known to set any pet's nerves off!  To prohibit this kind of behavior caused by this, spray some cat cologne on the base of your cats' tails.  This is where most of the scent glands are, which will give off the illusion that they have the same scent as eachother.
Both Max and Carlo may be completely healthy.  I do, however feel as though this type of behavior wouldn't be present unless something was off balance.  I would definitely have a wellness checkup for those two boys to be sure.
Again, I'm not sure if this is helpful, but these are the only things I have heard of that would cause this type of behavior.
Please keep me posted, and I apologize if this didn't help!
Sending tons of kisses to those furbabies!
Hilary