When cats who used to get along begin to fight there will be a good reason for it because cats behave instinctively to what is happening around them and what has happened to them in the past.
There are two leading possibilities for cats who at least tolerated each other and then suddenly one cat is aggressive towards the other, and these are (1) possible nonrecognition aggression or (2) redirected aggression.
This is a well discussed topic and it can happen suddenly but it should fade quickly and things should return to normal.
It occurs when the body scent of one of the cats changes. This may occur for a number of reasons such as being lost outside or being at a boarding cattery for a while or being bathed completely which removes a cat’s natural body odour. The other cat no longer recognises this new ‘stranger’ because domestic cats rely on their sense of smell tremendously. It is a very important tool of recognition, so important that if a cat cannot label a smell to a cat then that cat becomes hostile and a stranger. The result is aggression towards him or her. It’s instinctive and it is the raw wildcat coming out.
As mentioned, this problem should fade as the cat’s scent returns.
This is also a well-known phenomenon in the world of domestic cats. It happened to me recently when my cat bit my leg because he was wound up about the presence of a fox that he wanted to attack but couldn’t and therefore he attacked me. The bite happened just after the fox incident and was a clear case of redirected aggression.
Although it can happen in a number of possible ways. Another example is explained by Jackson Galaxy in his book Total Cat Mojo. Two cats are sitting side-by-side at a window looking outside. A stray cat walks in front of them and one of the indoor cats wants to chase and attack the wandering stray cat. He can’t but his instincts take over and without thinking rationally his desires overcome temporarily his friendship with the cat sitting next to him and he attacks him or her. This can damage the relationship between the two but I would hope that it would recover quite quickly.
This problem is harder to solve because it may not go away naturally if the event happens repeatedly. Jackson recommends trying to deter the stray cat from passing in front of the window either through TNR, blocking the window, removing the reason why the stray is outside the window or deterring the cat by various methods.
Humane cat deterrents and not very effective in my view. TNR might work if a local group of TNR volunteers are to hand and if the cat is a genuine stray feral cat but TNR should be done with caution because the cat might not be an unowned cat. Blocking the window is a last resort I would suggest because it’s not very pleasant both for the cats and their owner. However, it is possibly the best solution if the layout of the home allows it by which I mean the cats could watch out of a window from a different room to an exterior to which the stray cat has no access.