How did nations act to reduce the possibility of future wars after World War 1 and World War 2?

The world started the League of Nations after WWI. It proved to be a useless and worthless organization when Japan attacked China and Italy attacked Ethiopia in the 1930s, which eventually led to WWII.

After WWII, the world started the United Nations. One of the first crisis it dealt with was the Korean War which started in June 1950 when North Korea attacked South Korea. The United Nations immediately responded and South Korea was defended and is still a free nation today. The UN does not always respond to horrible events (Rwanda is an Excellent example), but, it does tend to help out those in need and has, (for the most part), done a pretty good job, since 1945.