After the World War I ended, the League of Nations was formed. Lot of nations became members of it, but not the United States. The United States felt that joining the League of Nations was not in its best interest, especially considering the fact that they wanted to further expand in the Pacific, nor were they willing to give up on the territories they recently gained and controlled. This changed after the World War II though, when the League of Nations became the United Nations, and the United States had different interests and were one of the first nations to join in.
