My Doberman backed a guy into the corner of my kitchen when he was in my house when I was gone. My niece was with this guy, coming to let Lyric out when he was an older pup still using a crate. Although he loved my niece, he didn't know that her b.f. was allowed in the house when I wasn't there. He may not have stuck with it if the guy had fought him, but most guys, if unarmed won't fight with a snarling, gnashing teeth Doberman whose muscles are flexed and he's backing them into a corner. Lyric never bit or got wild. But she said he stealthily moved this guy toward the door.
On another occassion, he ran in front of a guy who was about to knock on my door. He sat between the door (on the porch) and the guy and gave him a low, throaty warning growl. That guy decided to retreat with honor and go wait on the patio, away from the front porch. (this was all told to me later) Lyric wasn't afraid to get between the front door and the guy. But he also didn't fly off the handle and bite or get too riled up....just a little warning growl with one tooth showing. lol. I was in the shower and this guy, Lyric didn't know. When I came out to chat with the guy, Lyric was all friendly and accepting.
My latest GSD kept someone from coming in the house late at night. He heard someone messing with the door and came un-glued, gnashing teeth, snarling, barking. That guy, whoever it was went away.
The same thing happened with our Lab. I was so surprised because she LOVED people and didn't ever appear to have a problem with anyone coming around. But one night, when my teenage kids were at home alone and Bonnie was in my son's bedroom, she heard at the same time that my son heard the door handle jiggling. She ran downstairs and was VICIOUS at the front door. Vicious barking, snarling, all teeth. My kids were so scared and went onto the roof out my daughter's bedroom window and hid. They saw the shadow of a guy retreating down the long driveway.
When my husband and I came home, we looked at the door and the wood door jam was all screwed up. It looked like someone was indeed trying to get in. But they apparently changed their mind.
So, a dog doesn't have to engage in a fight to protect his owners necessarily. In most cases, the bad guy isn't going to try to fight the dog (unless he has a weapon) but rather, is going to think about breaking in, but changing his mind when he is confronted with examples like those.