As for the original question... My dogs all have an 'alert', maybe not guard, command - even the pomeranian mix because it gets them to quickly check out their surroundings and be ready.
What they do is go on alert - ears pricked, not TENSE, but not relaxed, and if they see what I'm trying to tell them to they will bark or vocalize. I use this to show them prey animals (flame me, my dogs are dogs, they rat for me and keep squirrels out of the property so they won't destroy my nut trees and ruin the gardens burying the nuts), if I am particularly uncomfortable with a person when I'm out with them I give the "Watch 'em" command, and the dogs pay attention.. the person catches this usually and makes note of not getting close. Keep in mind I'm about a 5'2 female in a questionable neighborhood and my dogs get walked mostly at night.. The command isn't to provoke aggression, but alertness, and if I tell them to, vocalizing. The vocalizing includes barking or growling/snarling.. The average Joe doesn't know an alert bark from a general dog's barking or vocalizing..
I taught it very simply (because all my dogs have good prey drive), I'd get all excited to excite them, show them the rat/squirrel ect. wait until they're focused and give the command "Watch 'em", praise, and release, they catch on very fast, all I have to do is say the command after the first few times. They start to catch on that it doesn't just mean fun little furry toys quickly, or at least mine have, because I'd give the command when there was only a person, no animal, around and they would eyeball the person and get praised.
My old bitch had it down so well if you said it she'd look all over and alert bark while still looking if nothing was around - she was always a bit ditzy though..
Not what I usually use it for at all, but a good example of why it comes in handy.. I had my elderly GSD pair out on unfenced property, they were 12 and 13, so slowing down and not the best hearing, eyesight, sense of smell.. I was a good 50' from them and they were coming up, sniffing and relaxing but following me. I saw - quiet to my horror - two labs that I know running loose. These two were a pair, a male/female, young, fit, and the most vicious dogs I've met to date. They spotted my two and started barreling towards them, I was going back to them as fast as I could and they still didn't know what was coming, I gave the alert command and they quickly looked around just in time to brace themselves. Those labs hit them like trucks and started biting immediately. At least my dogs were facing them and halfway ready to defend themselves. If the two had caught them, or two on one, when they were completely relaxed, loose and not expecting it and from behind, they would have been much worse off..
My dogs are little more than pets - but I've always had the command taught to my dogs, and my past dogs. It's never ever caused a problem. It's not teaching a dog to attack or bite, it's teaching them to go on alert.