Dog Body Language
Putting It All Together-The Whole Dog continued...
Offensively Aggressive
If your dog feels anger and confidence at the same time, you might see offensively aggressive body language. He’s on the attack, and he may or may not stop if the person or animal he’s focused on stays away or retreats. He does his best to look large and intimidating by holding his head high, his ears up and forward, and his tail raised and rigid. He might flag his tail. His hackles might be up. He positions himself over his forelegs so that he’s ready to lunge or charge forward. He stares directly at the person or animal. He shows his teeth by wrinkling his muzzle and retracting his lips vertically to display his front teeth. He growls, snarls or barks in a low, threatening tone.
Defensively Aggressive
Most dogs give plenty of warning before reacting aggressively, but you need to know what to look for to recognize the signs. If your dog is feeling defensively aggressive, he’d rather not get into an altercation if he doesn’t have to. He’d rather the person or animal he’s afraid of just back off and leave him alone. But at the same time, he’s ready to stand up for himself. Because he’s feeling both fear and anger, he often adopts a combination of fearful and offensive postures. Typically, he looks large, his ears are up and forward, and his tail is held high and rigid. He centers his weight squarely on all fours, over his forelegs or over his rear legs, depending on the situation.
Usually it depends on how close he is to the threat and whether his intention is to stand his ground, charge forward or retreat. Typically, he draws his lips back to display his teeth, and he may or may not wrinkle his muzzle. Usually he growls, snarls or barks, although his bark might be high-pitched. Often, his hackles are up. People sometimes refer to a defensively aggressive dog as adopting “a good offense as the best defense.” Dogs like this are sometimes bluffing in that they really would not fight if push came to shove-they would likely retreat. But other dogs will make the first strike, depending on the balance of confidence and fear they’re feeling.
WebMD Veterinary Reference from ASPCA Virtual Pet Behaviorist

