The ratification of the treaty of Versailles included that the US would join the League of Nations. Although the idea of a world league to keep the peace was actually the US Presidents idea, the US people themselves were tired of war and didn’t want to interfere any more in outside nations and their conflicts that would cost them the lives of US soldiers, so they didn’t want to join the League of Nations. Therefore the Senate rejected the ratification of the Treaty of Versailles.