In the name of stopping all future conflicts, they proposed that countries would join a League of Nations. Their true intention was for the League to serve as a framework for world government.

The U.S. didn't join because of the prevailing attitude after World War I. Don't get involved! Isolationism! Leave us alone! Non-interventionism was the word. We moved... We're oceans away from you assholes, so we didn't have to keep fighting these wars. No one wanted to be interested in European affairs in the United States. They're like, fuck it, we support this, but you could do it. I don't want to be a part of this.

The USSR didn't join because they were a totalitarian government and were not invited.

As for Germany, the rest of the world blamed Germany for starting World War I, so of course they weren't allowed to join the League of Nations when it was formed after World War I. Which was a bad idea!