1 I used to train police dogs in college. Just about any breed can be a good guard dog. ANy large dog can make a good police dog because to the dog, it's all a game. Any dog with high play drive can be taught.

2 Really? Just a very rough kind of play? I had no idea, that's interesting.

3 SayUncle is exactly right. To the dog any type of training is a form of play.

That's why you can get any dog to to almost anything if you frame in that reference.

There is even an excellent book out there called: Play Training Your Dog.

The military and to a certain degree most police and protection dogs are selected for characteristics that make them easily suited to the work: I.e they can be trained quickly.

Those dogs are often very high drive and very dominant something that makes them very challenging to have around the house as pets.

The drive means they will always want to be busy which means if you aren't "playing" with them they will find a way to amuse themselves. This often involves chewing things - lots of things.

The dominance means they will always challenge you for pack leadership so you have to be continually on your guard. This is where most people get into trouble.

Many military/police dogs are also quite stubborn and hard headed (GSD in particular) and this is why they make good candidates for that type of training - they tend to be more forgiving of "handler error" in the training process.

Dogs that wash out don't necessarily lack the potential to do the work - they just need a more patient approach and military/police forces don't have the time (or sometimes the training methods) to achieve that.

This is why there are more GSDs than Malinois in those applications eventhough the Mal is generally faster, smarter, and with higher drive. It is known for having a longer memory for "handler error" which leads to training issues.