Smart dogs know when to be aggressive and they use it sparingly. I had a collie cross mutt many years ago who could seem almost timid, certainly indifferent to people and dogs he didn't know. But one day he was in our yard bouncing around with the neighbor's two smaller dogs (his pals) and this guy walks by with a big German Shepherd unleashed. The shepherd suddenly turned and rushed our neighbor's dog and snapped at it, scaring it badly. The guy called his shepherd back(through much bellowing) and the dog went reluctantly, looking for more trouble. My dog Noah suddenly, silently took off like a shot and bowled the big shepherd over into a muddy ditch! The shepherd started screaming like he was shot, lol. Big bully. Noah came back when I said his name and stood there watching the guy fish his muddy dog back onto the road. So you see, a dog who backs down from most fights may well NOT back down when he perceives it as NECESSARY to protect someone.
This same dog knew exactly where our property line was and, if the kids were outside playing and grown men walked by on the road he would go out and pace alongside the men just four feet inside our property line staring at them. When a guy asked if he would bite, I said "Not if you stay on the road, but he probably will if you approach the kids." I didn't teach him to do this. He just picked the proper moments to be protective and I allowed that.
Let your dog grow up and you might be surprised at how good a dog can be at deciding when a fight isn't worth it and when a bit of posturing or even mild aggression is necessary. You have to let them get past being a teenager dog, though!