Reservationists: A group of senators known as "Reservationists" led by Henry Cabot Lodge raised concerns about the treaty. Lodge proposed a number of reservations to the treaty, including the rejection of Article 10, which committed the United States to collective security and military intervention in case of aggression against a member state of the League of Nations.
Isolationist sentiment: Many Americans, including some in the Senate, were opposed to the idea of the United States becoming entangled in European affairs and international alliances. They believed that the country should focus on its own domestic issues and avoid foreign commitments.
Political partisanship: The rejection of the treaty was also influenced by partisan politics. The Senate was controlled by the Republican Party, which was opposed to the Democratic President Woodrow Wilson, who had been instrumental in negotiating the treaty.
Lack of support from the American public: Public opinion in the United States was divided on the treaty. While some supported it, others were opposed or had reservations. The failure to secure broad public support weakened the treaty's chances of ratification.
As a result of these factors, the United States Senate ultimately voted against ratifying the Treaty of Versailles. The United States did not join the League of Nations and instead pursued a policy of isolationism in the years following World War I.