Why did the Senate, led by Henry Cabot Lodge, oppose U.S. membership in the League of Nations?

Henry Cabot Lodge feared that joining the League of Nations would create international alliances that could embroil the United States into future wars that would not serve America’s interests.
A vocal Republican opponent of the Democratic president, Woodrow Wilson, Henry Cabot Lodge was a strong isolationist who feared that the creation of the League of Nations, heralded by Wilson as a way to prevent future wars, would actually lead to future military conflict.
In a speech in Washington in August 1919 – only nine months after the Armistice that ended the Great War – Lodge cautioned that the United States needed to look after its own economic, political, and military interests first and, by doing so, would do more to further world peace than by joining the newly formed League of Nations.
Lodge’s opposition was instrumental in preventing the US from joining the League and, without US support, the League faltered in the following decades and failed to stem the forces which led to World War II.