My territorial dog fought my puppy. How to prevent future fights?

That sounds like a tough situation and I hate to hear this is happening. Most humans cannot correctly read canine body language without additional training. Unless you know what the dogs are saying to each other, it's impossible to know how to modify and train the dogs behavior. Canine aggression is a complex behavior as it can stem from dominance, possessiveness, fear, anxiety, resource guarding or a combination. I recommend to work with a professional trainer. This will give you the best and fastest results. Researching on your own will also be helpful. Familiarizing yourself with canine body language and dog training can help you to recognize what your pet is trying to say. You can find many books written on the topic and there is a trainer named Victoria Stilwell who I think is particularly helpful. Best of luck.

Your worry is very valid! It is best to make sure the two are only playing together when you can directly supervise them and that puppy and your older dog are separated when you are away so that an accident can't happen - even the most well intentioned of a dog can cause an injury! If the puppy starts playing too rough, a "time out" of a few minutes for everyone to calm down can also help and teaches your puppy that too rough of play is frowned upon. Diverting with treats and toys is also a great way, so keep that up.

I hate to hear this is happening. Unfortunately, it's impossible to know how best to advise you without being able to read the body language of the dogs. It's possible this is occurring because they are maturing. Both dogs also need to be spayed/neutered because this can help to decrease aggression. Intact dogs are much more likely to fight and have problems. Canine aggression is a complex behavior as it can stem from dominance, possessiveness, fear, anxiety, resource guarding or a combination. Most people are not familiar with reading canine body language and training. Because of this, I recommend to work with a professional trainer. Researching on your own will also be helpful. Familiarizing yourself with canine body language and dog training can help you to recognize what your pet is trying to say. You can find many books written on the topic and there is a trainer named Victoria Stilwell who I think is particularly helpful. Best of luck.

I recommend that you keep them separate until you are sure they can get along when you are gone. Separating by a baby gate or keeping them on leashes would be fine. If they fight over toys, then all toys should be removed from the area. You also. Red to keep a close eye on them around feeding time. Here is a helpful article https://www.animalhumanesociety.org/behavior/how-introduce-dogs

If your dog does not get along with other dogs it was probably a very bad idea (and a very stressful one) to bring a new puppy into the house. A new puppy can also be hard around a newborn as they aren't yet sure how to interact with each other. You can try to continue to get the two to get along by starting with limited introductions that are supervised, however if a fight breaks out the interaction should be stopped immediately and started again later. Never leave the two unsupervised together as even a good play sessions can cause accidental injury. You can also use baby gates as a way for the two to "meet" each other without being able to physically be together. If fights are continually breaking out, talking to a trainer or behaviorist that can observe within the home is best.

Many dogs experience resource guarding behavior, which leads them to become highly possessive over their food and/or prized toys or treats. The safest approach is to be sure that Freya and your puppy are separated in different rooms or crates while they have access to any food or treats. You can also contact a specialist in veterinary behavior (https://www.dacvb.org/) if this issue persists.

Your house is a lot different, and your own dog may be feeling like there is a bit of an "intruder" situation going on. it may take some time for your dogs to all get along, especially if the change was recent. Until they get along better, the dogs should only be together when able to be supervised by you. You may also want to pick up items of contention such as beds, toys, treats, etc that may be fought over if your dogs feel threatened. If a fight breaks out, immediately separate the dogs for a "time out"/cooldown, and then reintroduce when all dogs are calm and relaxed. When you are not at home, the dogs should be separated or crated in separate crates to avoid a potential fight. If the issue persists, speaking with a trainer or behaviorist may also help as they can often observe what is going on in the home, and can make suggestions based on your specific needs.