Most likely the answer you seek here is one regarding those in the United States who argued against the treaty. They feared it would force the USA to get involved in future international conflicts. There had been hesitancy by the USA to get involved in Europe's "Great War" (which later became known as World War I). Afterward, many in the USA wanted assurance that they wouldn't be dragged into another such overseas conflict again. US President Woodrow Wilson had proposed a document of 14 Points for the peace conference in Paris (that led to the Treaty of Versailles). One of Wilson's key points was the establishment of an international League of Nations to resolve disputes. But after the treaty was signed in Paris, back home in the USA, Wilson could not get agreement for the United States to become part of the League of Nations.
