Hello! You asked why the Versailles Treaty was defeated in the United States.
The Treaty of Versailles was actually the peace treaty which formally ended WWI. However, there were deep disagreements between the Big Four (the United States, France, Italy and the United Kingdom) at the Paris Peace Conference. While President Wilson had his Fourteen Points outlined and ready to be presented to European powers, he found that the leaders had other ideas regarding the peace. European leaders and President Wilson disagreed sharply regarding contested post-war redistribution of territories. Meanwhile, Russia was angered at being excluded from the Peace Conference: because it reneged on outstanding financial obligations to the Allied powers, the other European powers refused to recognize the new Bolshevik government and would not include it in peace negotiations. Germany was also angry because it felt that the treaty imposed extremely draconian punishments on her for starting the war. European powers were out for blood in the peace treaty, and Wilson was appalled that this was the case. However, further trouble was in store for Wilson on the home-front.
Wilson wanted a League Of Nations (similar to our United Nations today) set up, representing all interested nations, for the purposes of preventing future bloodshed and warfare. However, when he came back to the United States to present the treaty to his Senate, he was shocked that there were twin chasms of political opinion threatening the ratification of the treaty.
The 'Irreconcilables' refused to ratify the treaty at all. These were senators opposed to what they considered a threat to the sovereignty of the United States. They feared that the authority of the League Of Nations would supercede that of the United States when it came to the question of war powers; they did not want the United States to be subject to a foreign authority impeding the right to declare war on any nation which threatened its national interests. The 'Reservationists' were led by Henry Cabot Lodge, the Republican leader of the Senate: they would only ratify the treaty if certain conditions were met. It was Lodge who attached fourteen reservations or amendments to the treaty, which angered Wilson enough to request the Democrats to reject Lodge's amendments. The treaty went down to defeat on November 19, 1919 when Democrats joined the Republican Irreconcilables to reject the treaty. Some historians feel that personal enmity between Wilson and Lodge torpedoed the treaty. The United States never joined the League Of Nations; the Senate rejected the treaty 39-55.
Thanks for the question.