Shortly after the war, you have a significant Russian famine that kills five million people. Then in the early 30s, you have the Soviet famine, five to seven million people. This is believed to have occurred because of Stalin's attempts to make agriculture collectivized. In the late 30s, you have Stalin's purge, where he goes after political opponents, and it's believed that he killed as many as three million people. These things were so shocking to the planet that they made attempts to prevent them in the future. In 1920, out of the trauma of World War I, the League of Nations was founded. It was an attempt to prevent things like this in the future, for nations to talk to each other and to coordinate so they don't go to war, especially at the scale seen in World War I.