There are many signs that a dog is about to attack, such as barks. A wagging tail indicates an attempt to communicate excitement, but a territorial dog may wag its tail at a chance to defend its home. A highly disturbed dog may sometimes emit confusing or misleading signals, yelping or jumping.

Most human behavior (especially by people unfamiliar with dogs) can potentially evoke a predatory or aggressive response from some dogs. Not every dog responds to all or even any of these behaviors with aggression. However, some do. These behaviors include:

* Going near to fighting dogs or trying to break up the fight.
* Attacking a dog or its companions, or acting in a manner that the dog perceives as an attack (for example, a sudden enthusiastic hug or inadvertently stepping on any portion of the dog’s anatomy, such as a paw or tail).
* Attempting to take food away from a dog, or moving towards a dog’s food or between a dog and its food, even inadvertently.
* Threatening a puppy in the presence of an adult dog, especially its mother.
* Looking a dog directly in the eyes. In dog communication, this is an act of dominance or aggression. This is more dangerous when on the same visual level as the dog (such as small children), or when the human is unfamiliar to the dog.
* Approaching a sick or injured dog. Note that older dogs, like people, often become “cranky” and develop a tendency to become “snappish”.
* Related to the previous point, failure to recognize a dog showing signs of insecurity or fear and continuing whatever behavior is causing the dog’s anxiety to increase, until “fear biting” occurs. Again, an older or chronically infirm dog is liable to develop feelings of vulnerability and anxiety, and therefore become less tolerant and more aggressive.
* Running away from a dog: the atavistic chase-and-catch instinct is not fully lost, and most dogs can outrun and overtake the average human.
* Similarly, the natural instinct to jerk one’s hands upwards away from an inquisitive dog often elicits in the dog a strong impulse to grab and hold, or at least to investigate, resulting in the dog jumping on the person and thrusting its head towards the raised hands.
* Ignoring “Beware of Dog” signs: trained attack dogs, unlike most dogs, may attack an intruder without warning.
* Startling a resting or sleeping dog.
* Entering a dog’s “territory” and behaving in an unfamiliar pattern or being unfamiliar to the dog. The dog’s territorialism, powerful senses, and latent ferocity makes almost any dog, irrespective of size, a powerful deterrent to burglars. The territory that a dog recognizes as its own may not coincide with the property lines that its owner and the legal authorities recognize such as the inside of a neighbor’s home

Many adoption agencies test for certain aggressive behaviours in dogs, and destroy any animal that shows certain types of aggression.

source: wikipedia.org