Aggression in international law is defined as the use of force by one State against another, not justified by self-defense or other legally recognized exceptions. The illegality of aggression is perhaps the most fundamental norm of modern international law and its prevention the chief purpose of the United Nations. Even before the UN, the League of Nations made the prevention of aggression a core aim; and the post-World War II Allied tribunals regarded aggression as a crime under the rubric crimes against peace.

The most authoritative definition comes from the UN General Assembly. (The UN Charter never defines the term, instead banning the threat or use of force.) In 1974, it completed a twenty-year project to define aggression. Member States claimed that a definition would help the UN—principally the Security Council, charged by the Charter with addressing aggression—in responding more consistently and promptly. While it reflects a broad international consensus, it is not a treaty, though it may represent customary international law.

The definition begins by stating that “[t]he first use of armed force by a State in contravention of the Charter” constitutes prima facie evidence of aggression. The definition is somewhat limiting, and perhaps circular, in that the first use of force by a State would not be aggression if undertaken in a way consistent with the Charter. Thus, for example, the deployment of U.S. forces to Somalia in 1992, while the first use of force, would not be aggression because it was authorized by the Security Council under Chapter VII of the Charter. A number of States have accepted that a State’s first use of force to extricate its citizens from another State when they are in imminent danger, and the other State is not able to protect them, is not aggression (e.g., Israel’s 1976 Entebbe raid) and may be a form of self-defense. Some scholars and human rights activists have advocated a broader right of non-UN-approved intervention to prevent large-scale human rights abuses.

Moreover, most States seem willing to accept, albeit tacitly, a limited right of a State to a first use of armed force—so-called anticipatory self-defense—where it is facing a certain, imminent, and catastrophic attack, such as was faced by Israel on the eve of the June 1967 war. However, the assertion by the United States government in its 2002 National Security Strategy of a broader right of preemptive self-defense to deal with “rogue States and terrorists” possessing weapons of mass destruction elicited significant criticism internationally. Foreign governments and others also condemned the war in Iraq in 2003 in part because they rejected U.S claims that the action was authorized by prior Security Council resolutions (the U.S. did not make a legal argument that the war was taken as a matter of self-defense) and concluded that it was thus aggression under the Charter.
The General Assembly’s definition also offers an illustrative list of acts of aggression: invasion, attack, or occupation of whatever duration; bombardment; blockade; attack on another State’s armed forces; unauthorized 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 or substantial involvement therein.

Acts of aggression such as these trigger the two key lawful uses of force mentioned in the Charter: (a) individual or collective self-defense; and (b) force approved by the UN itself. Thus, the Iraqi invasion of Kuwait triggered the right of Kuwait and its allies to engage in self-defense, as well as the right of the UN to approve the use of force against Iraq under Chapter VII.

Despite its prohibition in international law, aggression remains a feature of international life. Although the classic attack on another State is not as rampant as before World War II, more subtle forms of armed intervention persist, and the responses of the international community remain plagued by timidity and inconsistency.

(See humanitarian intervention; just and unjust war.)