The much maligned League of Nations. It's criticised as ineffective, and failing to avoid World War II. But I think that is to expect way too much of any international organization, as we see with the United Nations today.
I've reading a little about the League lately. I was surprised to learn about how many international crisis there were beyond those most known (like the Italian invasion of Abyssinia, or Japan's push into China), For instance, Lithuania was torn between Polish and Soviet Russian armies. I didn't know that, but Vilnius, the capital of Lithuania, was actually occupied by Poland for most of the years between Lithuania's independence, and its absorption by the Soviet Union.