Aggression in human society is a highly complex phenomenon and covers a wide range of behaviour. It should be distinguished from aggressiveness. Aggressive behaviour can be considered to have a positive aspect in that it affords the natural drive to gain mastery over the external world, thus underlying the necessary attainment of individual independence and most of the greatest human achievements. It ordinarily fits the pattern of normal adjustment, while aggression tends towards maladjustment and abnormality, with behaviour that is essentially destruction oriented.
In a psychological sense, aggression refers to any manifestation of a self-assertive disposition. There is still considerable dispute as to whether it is simply a response to adverse external circumstances. It is nevertheless becoming increasingly apparent that particular styles of aggression relate to particular social contexts and roles and are thus culturally patterned.
In a political sense, aggression refers to any manifestation of an expansive policy; in a military sense to an unprovoked military attack; and in a legal sense to the use of armed force by a government in violation of an obligation under international law or treaty. In the last sense, the term has appeared in numerous treaties and official declarations since World War I, including the League of Nations Covenant and the United Nations Charter. In 1933, the signatories of the Convention for the Definition of Aggression agreed to define the aggressor in an international conflict as that state which is the first to commit any of the following actions: (a) declaration of war upon another state; (b) invasion by its armed forces, with or without a declaration of war, of the territory of another state; (c) attack by its land, naval, or air forces, with or without a declaration of war, on the territory, vessels, or aircraft of another state; (d) naval blockade of coasts or ports of another state; (e) provision of support to armed bands formed on its territory which have invaded the territory of another state, or refusal, notwithstanding the request of the invaded state, to take on its own territory all the measures in its power to deprive those bands of all assistance or protection. Furthermore, it was stipulated that no political, military, economic, or other considerations could serve as an excuse or justification for such acts of aggression.
Man is the only vertebrate who destroys members of his own species. No other animal takes positive pleasure in the exercise of cruelty upon another of his own kind. The extremes of 'brutal' behaviour are confined to man and there is no parallel in nature to our savage treatment of each other.
The problem of war is more compelling than ever before in history and is compounded by the fear of destruction by nuclear weapons. The complexities of the circumstances which provoke war are such that no one man and no one viewpoint can possibly comprehend them all. One such complexity is the fact that instances of violations of pledged words are innumerable in the diplomatic and military history of the twentieth century. If stability in world affairs is ever to be achieved, the psychological point of view deserves equal consideration with the political, economic and other aspects. The study of human aggression and its control is, therefore, relevant to the problem of war although, alone, it cannot possibly provide a complete answer.
Aggression is a necessary part of the biological inheritance of mammals birds and insects with which humans have to learn to co-exist. It has served and continues to serve to preserve us.