The Kellogg-Briand Pact was drafted in 1928. The pact prohibited the nations who signed it from going to war. It was named after its two authors; the Secretary of State for the United States, Frank Kellogg and Aristide Briand the Prime Minister of France.
The pact took practical effect after World War II. The Nazi war criminals were tried and hung in 1946 for violating it. Since then formal declarations of war have been very rare in the world.