The League of Nations was the first attempt at something like the United Nations that we now have. It was formed after WWI. It was meant to keep the peace in the world in two ways. First, it would give countries a place to talk about their differences. Second, if a country attacked another country, all the other countries were supposed to go to war with the country that attacked (this is called "collective security").
The League of Nations failed for a couple of reasons:
So it quickly became obvious that the League of Nations wasn't accomplishing anything and it fell apart.