Why is my dog attacking my other dog all of a sudden?

If one of your dogs suddenly attacks another, that shock is immediate and disorienting. Read this as a practical road map: who this applies to, quick likely causes, what to look for right away, how to keep everyone safe, and steps to rebuild a stable household. If the situation is acutely dangerous, follow the safety steps below and contact emergency help before attempting long-term changes.

Who this guide helps — owners, sitters, and trainers who need clear answers

This guidance is aimed at people who live with more than one dog and are facing a sudden change in inter‑dog behavior. It is especially useful when a household has had a recent adoption or change — a new dog, new baby, visitors, or a move — because those shifts often alter day‑to‑day tension in ways owners don’t expect. It also applies when two dogs who were once bonded start fighting for the first time, or when an owner notices aggression following a vet visit, illness, or surgery; pain and discomfort commonly change how a dog tolerates other dogs. Finally, families with children or multiple “resources” (food, beds, laps) may see fights flare in unpredictable moments. I typically advise owners in these situations to prioritize immediate safety, document what happened, and get a vet check if the onset is sudden and unexplained.

Short take: the most common reasons dogs suddenly attack one another

• Resource guarding — Dogs may guard food, toys, resting spots, or a favored person. A previously tolerant dog can become possessive when competition or stress increases.
• Redirected or triggered aggression — A dog that is aroused or frustrated by an outside stimulus (another dog seen through a fence, a loud noise, a strange person) may redirect that arousal onto a household mate.
• Pain or medical change — New pain, ear problems, dental pain, neurological events, or medication side effects can make a dog irritable and more likely to snap.
• Social tension — Changes in hierarchy, fear, anxiety, or poor social skills between dogs can build until a threshold is reached and aggression appears.

Body language, hormones, and instinct — how dogs communicate aggression

Dogs communicate constantly with posture, gaze, ear and tail position, facial tension, and the speed of their movements. What looks like “sudden” aggression is often the end of a gradual escalation: subtle warnings like stiffening, hard eye contact, fixed position over a resource, or mounting postural changes are often missed. Because dogs are tuned to small shifts in body language, one dog’s polite signal being ignored can lead to louder signals and, eventually, a bite.

Hormones and reproductive status can also shift behavior. Intact animals or dogs in households with an intact female in heat may show increased arousal and competition that is likely linked to underlying reproductive drives. Older dogs experience hormonal changes that may reduce impulse control.

Medical issues frequently change temperament. Pain, vestibular or neurological problems, and metabolic disturbances can make a dog less tolerant of proximity or touch; a dog suddenly aggressive toward a housemate may be telling you it hurts. Stress physiology also plays a role: chronic stress increases cortisol and baseline arousal, which reduces the margin of safety for self‑control. Under high arousal, even a mild trigger can push a dog past their threshold.

Environmental sparks: household and routine triggers to watch for

Most fights follow recognizable patterns if you look for them. Resource hotspots are common flashpoints — feeding bowls, prized beds, the doorway, and laps during petting are small spaces where competition concentrates. Observe who arrives first and who challenges; small changes in routine (moving a bed, changing feeding order) can flip the balance.

Recent environmental changes are key. A new person, pet, or daily schedule can shift where dogs focus their attention. For example, a new baby or guest may pull a favored person’s attention, increasing guarding behavior. Even a remodel or moving a couch can change preferred resting spots and trigger conflicts.

High‑arousal contexts often reveal poor tolerance: walks, greetings with visitors, or play with a third party can raise arousal to a point where one dog redirects onto another. Time patterns matter too: some dogs are more reactive after exercise when adrenaline is high, others in the evening when they are tired and less patient, and females may show different tolerance during estrus cycles.

Recognizing danger — red flags that demand immediate intervention

• Uncontrolled, escalating or repeated severe bites — any event that produces deep wounds, prolonged fighting, or the need for surgical repair requires immediate veterinary and behavioral help.
• Sudden onset in a well‑mannered dog — a sharp change in behavior with no clear trigger may suggest an underlying medical problem that needs prompt attention.
• Signs of serious injury, shock, disorientation, or neurological deficits — stumbling, head tilt, collapse, or loss of bladder/bowel control after an incident necessitates emergency care.
• Owner inability to safely separate dogs — if you cannot separate them without risking human injury, call for professional assistance (vet, emergency clinic, or behaviorist who can guide you).

What to do right now: safe first steps for owners to de‑escalate the situation

After an incident, create a calm perimeter. Use barriers such as baby gates, doors, or crates to separate the dogs without physically grabbing at them, which can lead to owner injury and further arousal. If separation has to occur in the moment and both dogs are still engaged, put a barrier between them (a large board, a sheet draped over them) rather than trying to insert hands. If you must grab collars, do so cautiously and only if you can safely do so.

Check each dog for injuries and document what you see — photographs of wounds, the location, and time can be important for your vet or a behavior consultant. Even small punctures can become serious. Avoid scolding or using physical punishment: that often increases fear and arousal and makes the problem worse.

Contact your veterinarian promptly to rule out medical causes. I typically recommend a medical exam before making any long‑term behavior plans when a once‑friendly dog suddenly becomes aggressive. If injuries are severe, seek emergency veterinary care. If the situation is stable but you’re worried, call a certified behavior professional for triage guidance before attempting reintroduction.

Long-term fixes: training approaches and home management to prevent fights

Long‑term reduction in inter‑dog aggression rests on three pillars: lowering immediate competition, rebuilding positive associations between the dogs, and teaching impulse control. Start by managing resources: feed dogs separately (in different rooms or crates), rotate toys, and provide clearly defined resting areas so a dog doesn’t have to defend a prized place. Consistent routines reduce uncertainty and background stress.

Controlled reintroductions should be gradual and done on neutral ground if possible. Begin with parallel activities — walking the dogs on either side of a neutral handler at a distance where both dogs are calm. Reward calm behavior with high‑value treats and praise. Slowly reduce distance over multiple sessions, never allowing interaction at the point of obvious tension. If thresholds are crossed, back up and reduce challenge.

Desensitization and counter‑conditioning can change an emotional response. For example, if one dog stiffens when the other approaches a bed, have the stiff dog earn a treat for staying relaxed while the other eats at a distance, gradually lowering the distance as comfort increases. Teaching impulse control cues — “wait,” “place,” “leave it,” and a strong settled cue — helps dogs gain predictable structure and reduces spontaneous conflict.

When to involve a professional: engage a certified behaviorist or an experienced positive‑reinforcement trainer whenever bites have occurred, injuries are present, or when you can’t safely manage separation and training on your own. A behaviorist will assess medical history, observe interactions, and design a rehabilitation plan with measurable steps. I typically see the best outcomes when owners are consistent, patient, and willing to change the environment as well as the dogs’ behavior.

Practical gear and safety tools — what helps during prevention and emergencies

Practical tools support safety and training without causing harm. Baby gates and sturdy crates let dogs be separated safely while still allowing visual contact in some cases. Crate training can provide each dog with a predictable safe space. Use properly fitted muzzles as a temporary safety measure when introducing hands-on medical care or when supervised work is being done under professional guidance; muzzles must be introduced slowly and associated with positive experiences so the dog is not further stressed.

Secure, well‑fitted leashes and front‑clip harnesses give handlers better control during walks and reintroductions. Avoid devices that cause pain or fear — shock collars and prong collars often increase anxiety and may worsen inter‑dog aggression unless used by a professional within a carefully controlled program. If bite risk is high, consult a behaviorist about the safest tools and protocols to use.

Sources and further reading from vets and behaviorists

• Merck Veterinary Manual — Aggression in Dogs (section on diagnosis and management of aggressive behavior)
• American Veterinary Society of Animal Behavior (AVSAB) — Position Statements and resources on dog behavior and inter‑dog aggression (avsab.org)
• American College of Veterinary Behaviorists (ACVB/DACVB) — Find a boarded veterinary behaviorist directory and clinical guidance (dacvb.org)
• Journal of Veterinary Behavior — Clinical studies and reviews on interdog aggression and behavior modification techniques
• Merck Veterinary Manual — Pain and Behavioral Changes in Dogs (sections linking medical disease and temperament)