It may seem like common sense, but there is a right way and a wrong way of introducing two dogs. Any time two dogs are meeting for the first time, especially if one of the dogs is going into the other dog’s house, it’s important to introduce them properly.
First, the introduction should be on neutral ground, outside of the house. Second, they should get accustomed to each other’s presence by walking together at a safe distance apart, say across the street from each other. If one of the dogs is reactive, that dog should follow the non-reactive dog. (If you have a reactive dog, this process can be a complicated so please seek guidance from a professional before you try it by yourself.)
Always watch for body language. If at any time either dog starts displaying signs of stress such as staring, stiff body posture, ears back, that means they are too close together. Increase the distance between until they are comfortable and walk some more.
Once they are comfortable enough with each other for a sniff, never allow it to be more than a few seconds. Lure them out with food or a hand target. Never pull! Never have a tight leash when they are greeting each other! The leash should always always always be loose and dangling. Tight leashes can cause aggression.