According to Wikipedia, the Kellogg-Briand Pact passed in 1928 outlawed war. A Peace organization has an annual contest to help fulfill the promise of the pact.
The Kellogg–Briand Pact (or Pact of Paris, officially General Treaty for Renunciation of War as an Instrument of National Policy) was a 1928 international agreement in which signatory states promised not to use war to resolve “disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them”. Parties failing to abide by this promise “should be denied of the benefits furnished by this treaty”. It was signed by Germany, France and the United States on August 27, 1928, and by most other nations soon after. Sponsored by France and the U.S., the Pact renounced the use of war and called for the peaceful settlement of disputes. Similar provisions were incorporated into the UN Charter and other treaties and it became a stepping stone to a more activist American policy. It is named after its authors, United States Secretary of State Frank B. Kellogg and French foreign minister Aristide Briand.