Making a dog aggressive means immense stress caused most commonly by fear with territorial or dominance issues coming second and third.
If you want to train your dog to be aggressive towards strangers you should think of other ways to protect yourself first and only look into protection dogs once all other options are exhausted.
No more fear
Dog aggression can have several causes like traumatic events for rescues, lack of exposure. Record your dog's body language, not the circumstances, and approach a professional if you're unsure.