Well, this is what it can feel like for the resident dog when a newbie four-legger enters into his world.
Instead of the nuclear approach of bringing home a newbie and expecting the dogs to organically get along, try a different tack. By making gradual introductions and protecting the existing dog's property and personal space, the two dogs are much more likely to hit it off rather than fight.
Instead of the stranger invading your home, imagine a pleasant day trip to the beach. While swimming in the surf you encounter a like-minded individual and strike up a conversation. You get along so well that you spend the afternoon chatting and before you know it, it's time to go home. You're reluctant to part and are about to exchange phone numbers when your partner invites your new friend home for supper. Instead of the sadness of parting, you experience the joy of more time together.
See the difference?
In terms of two dogs accepting one another, those early meetings are crucial for setting the tone of the future relationship.
Acceptance is most likely to occur when there is a clear pecking order between the two dogs. This is because it is tussles for supremacy that are most likely to undermine their relationship. Since the new dog is entering into the established dog's territory, it is only fair to have your dog treated as if he's in charge with the newbie in a subordinate role. When the dogs receive this message loud and clear, they will get along together just fine.
In addition, it's helpful to have: