Redirected aggression is one of the most frightening things that happens between cats who normally get on, because it looks like a friendship has suddenly collapsed. It has not. One cat sees a strange cat outside, her arousal rockets past anything she can discharge at the real target through glass, and the charge lands on whoever is nearest: often her housemate, sometimes you. The correct response is about arousal and time, not about blame, apologies or making the cats confront the stranger together, which would pour fuel on exactly the wrong fire.
In the Moment: Separate Without Using Your Hands
Never reach between fighting cats, never pick up the aroused cat, and never carry one cat to look at the other or at the window. Slide something solid between them, a door, a large cushion, a thick blanket dropped over the aroused cat, or herd one cat gently into another room with a barrier and close the door. Move slowly, speak low, and accept a few dramatic minutes rather than grabbing anyone, since redirected bites aimed at hands are how owners end up needing medical care. Check both cats afterwards for bites and scratches, looking through the fur with your fingers as well as your eyes, because punctures hide and cat bites infect easily; any wound, lameness or later lethargy earns a vet call. Then leave the cats alone.
Let Arousal Fall Fully, in Hours Not Minutes
A cat can stay physiologically primed long after she looks calmer, which is why reunions staged after twenty quiet minutes so often explode on contact. Give the aroused cat a proper cool-down in a single room with food, water, a tray and somewhere to hide, measured in hours and sometimes overnight, and judge readiness by the whole picture: normal breathing, ears and tail in their usual positions, eating readily, and showing no fixation on the door. Only then begin a staged reunion: scent first, by swapping bedding or rubbing each cat with the same cloth, then sight through a cracked door or barrier with food served on both sides, then short shared time, supervised, ending before any tension builds. If either cat stiffens, growls or fixates at any stage, go back one step and give it more time. Repeated blow-ups after window incidents deserve a full formal reintroduction over days, exactly as if the cats were strangers, plus a vet conversation about pain or anxiety lowering one cat's threshold.
Manage the Window Trigger Itself
While the cats rebuild, reduce how often the trigger fires. Note when the strange cat appears and which glass it uses, then screen the lower part of that window with removable film, a blind drawn at visiting hours, or furniture moved to block the prime viewing perch. Do not let your cats out to confront the visitor, and do not hold them up to see it off; both raise arousal and risk a real fight. Outside, remove invitations where you can: no food left outdoors, and motion-activated deterrents placed to discourage visiting cats from your garden are fair game. If the stranger is a neighbour's cat, a friendly word about its visiting pattern sometimes produces a timetable truce. Each change lowers the number of rehearsals, and rehearsals matter: every explosion deepens the pair's association of each other with danger.
The Household Rules That Prevent a Repeat
Agree the plan as a household before the next incident, not during it: who moves which cat, which room is the cool-down room, and the rule that nobody reunites them early out of optimism. Keep a brief log of incidents, what was at the window, how long separation took, and how the reunion went, since a rising frequency or a lengthening cool-down changes the advice you need. Tell every member plainly that punishment plays no part here; a cat punished mid-arousal learns only that the household is also dangerous. The log and the agreed steps are easiest to keep where everyone can see them, and KinsLe gives you that shared space when the plan is written into Care Teams notes.