The Kellogg-Briand Pact, also known as the Pact of Paris, is a treaty between the United States and other nations "providing for the renunciation of war as an instrument of national policy."

It was proposed in 1927 by Aristide Briand, foreign minister of France, as a treaty between the United States and France outlawing war between the two countries. Frank B. Kellogg, the U.S. Secretary of State, responded with a proposal for a general pact against war.

After negotiations, it was signed in Paris on August 27, 1928 by eleven states: Australia, Canada, Czechoslovakia, Germany, India, the Irish Free State, Italy, New Zealand, South Africa, the United Kingdom, and the United States. Four states added their support before it was proclaimed—Poland, Belgium, and France (in March), and Japan (in April). It was proclaimed to go into effect on July 24, 1929. Sixty-two nations ultimately signed the pact.

Because the Kellogg-Briand Pact was concluded outside the League of Nations, it did not perish with the League. The pact is a binding treaty under international law and, from a technical legal point of view, it remains in force as part of the supreme positive law of the United States, under Article VI of the United States Constitution.

As a practical matter, the Kellogg-Briand Pact did not live up to its aim of ending war, and in this sense it made no immediate contribution to international peace and proved to be ineffective, especially after the Japanese invasion of Manchuria in 1931 and the Italian invasion of Ethiopia in 1935. However, the pact is an important multilateral treaty because, in addition to binding the particular nations that signed it, it has also served as one of the legal bases establishing the international norm that the use of military force is presumptively unlawful.

Notably, the pact served as the legal basis for the creation of the notion of crime against peace -- for committing this crime, the Nuremberg Tribunal sentenced a number of persons responsible for starting World War II.

The interdiction of aggressive war was confirmed and broadened by the United Nations Charter, which states in article 2 paragraph 4 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." The consequence of this is that after World War II, nations have been forced to invoke the right of self-defense or the right of collective defense when using military action and have also been prohibited from annexing territory by force, although at times these justifications may seem by some to be straining credulity.