Answer:
The league was established at the end of world war 1 as an international peacekeeping
The most obvious answer surely is language.
Language enabled people to communicate in the earliest of days, when mankind was still in its infancy. Language made communication between different societies faster and most probably contributed a lot to the quick spread of our species around the planet; especially the quick technological advancements we have witnessed in the past.
The event that caused the United States to enter World War 2 was the Japanese bombing of Pearl Harbor.
