An examination of the skillful political maneuvering of William Borah and Hiram Johnson, two of the post-war leaders of Republican progressivism, this study analyzes efforts to prevent U.S. entry into the League of Nations despite overwhelming support for the organization among both Democrats and Republicans. Following the debacle of the 1912 election, the leadership of the Republican Party embarked on a strategy of reconciliation designed to end the acrimony between progressive and conservative factions so that it could unite against the Democratic Party. A small group of progressive Republicans quickly realized that they could threaten to resume infighting and could, thus, influence policy making on important foreign policy issues. This political environment enabled William Borah and Hiram Johnson to have an extraordinary influence over the Republican Party's position concerning the League of Nations, an organization which they regarded as an agency for the perpetuation of European empires and, therefore, a threat to American democracy. Borah and Johnson effectively intimidated their party leadership and blocked the American participation in the League. Once this pattern was established, it would continue to influence Republican Party actions, in particular the construction of the Republican Party platform in 1920, the U.S. position regarding the Washington Conference on Naval Disarmament, and the issue of U.S. membership in the World Court.