To look at the Bernese mountain dog you would think that it is a soft, gentle dog with a fun sense of character. Whilst this is mainly the case, the breed was originally bred to be a watchdog and they still have that same instinct embedded in them today.

The History of the Bernese as a Watchdog and How it Compares Today

The Bernese mountain dog has come a long way since it was first introduced into the world. Its main purpose in Roman times was to guard cattle and property. It was a mastiff type dog and it did have a tendency to become aggressive if provoked. This was only bred out of the dog when it was needed more for pulling carts over the Alps from village to village.
As time went on, the need for the Bernese to be aggressive and watchful died down. These days they are still sometimes watchful but generally the breed is much friendlier. They love being around people and they are really gentle around their family. There is still the ability to make the breed guard things however, but generally it is not recommended.
Instinct tells the Bernese mountain dog to guard. It is naturally wary of strangers and it will bark to warn them off. However, it will not usually bite and attack a stranger; it is far too gentle to do that. Unless the dog has had no training at all, it will be a loyal, friendly and caring companion. So whilst there is the potential there for the breed to be a guard dog, it is usually not going to do it unless you have not trained it.
If you would still like to use the Bernese as a guarding dog then really you may have to compromise. The breed will make a good watchdog, but they will not make good guarding dogs. So you could easily teach your Bernese to watch over your home and to bark when any strangers approach. You cannot however train them to be overly aggressive. Usually a bark from a dog the size of the Bernese mountain dog would scare any potential stranger away however. So using them as a watchdog would be just as effective as using them as a guarding dog.
Overall the Bernese is much more of a loyal, gentle companion which loves nothing more than to be a part of the family. It will bark to warn off strangers, but it will not be nasty to them unless they are provoked. So keep this in mind if you are looking for a guard dog. It would be a much better idea to use the breed as a watchdog if anything, not a guard dog.