I would also look around your home/yard for anything that may be a trigger to guarding/defense mode. Initially eliminate things like food, water, treats, toys, etc. I've also found dogs to be protective of their bed/crate etc. These things may be able to be added back in safely later.
I also agree with the comment that it can be difficult and trigger aggression if one dog is leashed and one is not. The leashed dog suddenly has a very small territory, is forced in a position between you (the loved and protected owner) and the unleashed dog, and if fearful or feeling threatened this can be heightened because they feel they cannot get away to safety.
If there is a way to make introductions without you in the middle this may help too. A dog that is highly bonded to a person is not always willing to accept a new dog near that person initially.
I also agree with the comment that it can be difficult and trigger aggression if one dog is leashed and one is not. The leashed dog suddenly has a very small territory, is forced in a position between you (the loved and protected owner) and the unleashed dog, and if fearful or feeling threatened this can be heightened because they feel they cannot get away to safety.
If there is a way to make introductions without you in the middle this may help too. A dog that is highly bonded to a person is not always willing to accept a new dog near that person initially.