A crime against peace, in international law, consists of starting a war. For committing this crime, the Nuremberg Tribunal sentenced a number of persons responsible for starting World War II. One consequence of this is that nations who are starting an armed conflict must now argue that they are either exercising the right of self-defense or the right of collective defense. It has made formal declaration of war uncommon after 1945.

In 1927, the Kellogg-Briand Pact, known as the General Treaty for the Renunciation of War, said:

The High Contracting Parties solemly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another.

The United Nations Charter says in Article 1:

The Purposes of the United Nations are:

1. To maintain international peace and security, and to that end: 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, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace;
2. To develop friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples, and to take other appropriate measures to strengthen universal peace;

The interdiction of aggressive war was confirmed and broadened by the United Nations' Charter, which states in article 2, paragraph 3 that

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, or in any other manner inconsistent with the Purposes of the United Nations.

Article 33

The parties to any dispute, the continuance of which is likely to endanger the maintenance of international peace and security, shall, first of all, seek a solution by negotiation, enquiry, mediation, conciliation, arbitration, judicial settlement, resort to regional agencies or arrangements, or other peaceful means of their own choice.

The Security Council shall, when it deems necessary, call upon the parties to settle their dispute by such means.

Article 39

The Security Council 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.

See also : International Criminal Court.