It is both possible and desirable, with a view to ensuring international peace and security and to developing international criminal law, to define aggression by reference to the elements which constitute it.
Since aggression is the most serious and dangerous form of the illegal use of force, being fraught, in the conditions created by the existence of all types of weapons of mass destruction, with the possible threat of a world conflict and all its catastrophic consequences, aggression should be defined at the present stage.
The adoption of a definition of aggression ought to have the effect of deterring a potential aggressor. It would simplify the determination of acts of aggression and the implementation of measures to suppress them and would also facilitate the protection of the rights and lawful interests of, and the rendering of assistance to, the victim.
One of the fundamental purposes of the United Nations is to maintain international peace and security and to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace.
The Security Council, in accordance with Article 39 of the Charter of the United Nations, shall determine the existence of any threat to the peace, breach of the peace or act of aggression and shall make recommendations, or decide what measures shall be taken in accordance with Articles 41 and 42, to maintain or restore international peace and security.
Article 1 of United Nations General Assembly Resolution 3314 (XXIX), Definition of Aggression, states; "Aggression is the use of armed force by a State against the sovereignty, territorial integrity or political independence of another State, or in any other manner inconsistent with the Charter of the United Nations."
The first use of armed force by a State in contravention of the UN Charter constitutes prima facie evidence of an act of aggression although the Security Council may, in conformity with the Charter, conclude that a determination that an act of aggression has been committed would not be justified in the light of other relevant circumstances, including the fact that the acts concerned or their consequences are not of sufficient gravity.
Any of the following acts, regardless of a declaration of war, qualify as an act of aggression: (a) The invasion or attack by the armed forces of a State of the territory of another State, or any military occupation, however temporary, resulting from such invasion or attack, or any annexation by the use of force of the territory of another State or part thereof, (b) Bombardment by the armed forces of a State against the territory of another State or the use of any weapons by a State against the territory of another State; (c) The blockade of the ports or coasts of a State by the armed forces of another State; (d) An attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State; (e) The use of armed forces of one State which are within the territory of another State with the agreement of the receiving State, in contravention of the conditions provided for in the agreement or any extension of their presence in such territory beyond the termination of the agreement; (f) The action of a State in allowing its territory, which it has placed at the disposal of another State, to be used by that other State for perpetrating an act of aggression against a third State; (g) The sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to the acts listed above, or its substantial involvement therein (Article 3 of United Nations General Assembly Resolution 3314 (XXIX), Definition of Aggression).
Aggression was described in the International Military Tribunal Judgement at Nuremberg as "the supreme international crime." It further affirmed that crimes are not instigated or committed by abstract entities but only by people. Any individual – regardless of rank or station – responsible for "planning, preparation, initiation or waging a war of aggression..." was guilty of a Crime against Peace.
A war of aggression is a crime against international peace. Aggression gives rise to international responsibility.
Whenever governments are called upon to decide on the existence or non-existence of 'aggression under international law', they base their judgement on criteria derived from the 'natural', so to speak, notion of aggression, and not on legal constructions. This "natural notion" of aggression is a "concept per se", which "is not susceptible of definition". A 'legal' definition of aggression would be an artificial construction", which could not be comprehensive enough to comprise all imaginable cases of aggression, since the methods of aggression are in a constant process of evolution.