This study examines the roles played by Elbridge Durbrow and Frederick Nolting, U.S. ambassadors to South Vietnam from 1957 to 1961 and 1961 to 1963, respectively, in the implementation of U. S. policy at a pivotal moment when Washington conceded some of the leverage that it might have wielded over South Vietnamese president Ngo Dinh Diem. It illustrates how presidential administrations redefine ambassadorial roles and how ambassadors carry out their instructions. Both ambassadors acted in ways that officials in Washington may not have appreciated or expected, and therefore the study stresses the contribution of the embassy to policy development. The ambassadors who were posted to Saigon prior to the introduction of U.S. combat troops occupied a critical post for the implementation of policy. Ultimately, the manner in which Nolting played the role that the Kennedy administration scripted for him increased the odds that the only way of dealing with Diem would be through his ouster.