Versailles
Since we are playing "what if?"...
What if the Treaty of Versailles had not ground Germany into the dust and caused excessive suffering in Germany during the Great Depression? What if the Weimar Republic had not fallen as a result of economic suffering and ethnic humiliation caused by Versailles? What if the United States had joined the League of Nations and had acted as an active partner in diplomatic and economic cooperation with other nations instead of withdrawing into isolation?
What if the citizens of the world had behaved with compassion, generosity and forgiveness to each other instead of with suspicion, selfishness and vengeance after World War I?
We can take this back as far as you want.