at the end of World War 1, why did the united states senate ultimately vote against the ratification of the treaty of versailles?

Answer 1

To answer this question think about how most Americans felt about World War I as it began. Americans at the end of the war wanted to return to their pre-war stance on foreign affairs. To understand why they rejected the Treaty of Versailles, and as an extension the League of Nations, you would need to understand pre-war sentiments.