What it is: Aggression is a first act of hostility by an individual or group against another individual or group, and often refers to the first use of armed force by one state against another. It is not the same as aggressiveness.

What it means:
The first use of force (aggression) is banned by the United Nations. Article 2 of its Charter, drawn up in 1945, says 'All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State'. Under international law, aggression is illegal, though it took the United Nations General Assembly 20 years to agree on a definition of it. This lists acts of aggression, including invasion, attack, or occupation; bombardment; blockade; attack on another State's armed forces; unauthorised use of military forces stationed in a foreign State; allowing territory to be used for aggression; and sending armed bands or similar groups to carry out aggression.

Think about it: Under the UN Charter there are 2 uses of force which are legal: (a) self-defence and (b) force approved by the UN. Some political and military leaders think that aggression itself can sometimes be a form of self-defence (especially if it is their country which is making the first move).  The United Nations 'solved' this by requiring that any such act of aggression must be clearly sanctioned by the UN. Arguments about whether the US/UK attack on Iraq in 2003 was illegal aggression or legal self-defence were heated and prolonged. Some military actions have been called 'armed interventions' and other names in order to claim legality. Leaders determined to make acts of aggression have looked for loopholes to make them legal - including making sure something happens that looks like aggression against them and thus provides an excuse. So just how strong is that other force - the force of international law?  Can it be strengthened? Should it be strengthened? Are there other ways of peaceably preventing acts of aggression? Are there ways that haven't yet been tried?
See also:
Seville Statement | War only an invention | Milgram experiment | Robber's Cave | Prisoner's Dilemma | Christmas Truce |