Guarding without aggression? I think it is not only possible, but it is also how my bullmastiff seems to do it. Two times while on leash my female bully has felt threatened. Both times she used her body to push me away from what she percieved to be the threat, while giving a soft gruff warning bark as she retreated (with me in a forced retreat :f ). On one other ocassion a friend of mine (who she didn't know) abruptly entered our house. All she did was position herself in the room between me and him and bark loudly while holding her ground. In every case she demonstrated what I would call gaurding, without displaying any aggression at all.