Final answer:
Republican US senators opposed the Treaty of Versailles due to concerns over the harsh treatment of Germany, exclusion from negotiation by Wilson, and fears over loss of sovereignty and entanglement in foreign conflicts due to the League of Nations.
Step-by-step explanation:
Republican US senators opposed the Treaty of Versailles for several reasons. Many Republicans, known as the Irreconcilables, opposed the treaty outright, while others, the Reservationists, were open to the treaty if certain amendments were made. Key reasons for opposition included a belief that the treaty was too harsh on Germany, President Wilson's failure to include Republicans in the negotiation process, concerns over the League of Nations infringing upon US sovereignty, and the potential for the US to be drawn into foreign conflicts against congressional wishes.
The main contentions revolved around Article X of the League of Nations, which many senators feared would commit the US to international military action without Congress's explicit consent, thus overriding their constitutional power to declare war. Senator Henry Cabot Lodge led the conservative opposition, concerned about both the loss of US sovereignty and the potential for unchecked foreign entanglements. The controversy and lack of consensus resulted in the US Senate ultimately rejecting the treaty, preventing US membership in the League of Nations and leading to an isolated stance in international affairs.