The New Zealand Labour Party's election victory on 14 July 1984 resulted in an official rejection of the global strategy of nuclear deterrence. This action was the most fundamental challenge to the defence relationship between New Zealand and the United States since the signing of the ANZUS Treaty on 1 September 1951. This thesis is concerned with the effect of Prime Minister David Lange's personality on the resulting dispute between the two nations. This qualitative study utilises a theoretical framework articulated by Margaret G. Hermann which seeks to demonstrate the relationship between the idiosyncratic characteristics of leaders and the foreign policy behaviour of their respective nations. In order to effectively conduct this study, a number of key individuals involved in various aspects of the ANZUS dispute were interviewed by this author. It should be noted that David Lange was seriously ill throughout the course of this study and was unable to be interviewed by the author. Sir Geoffrey Palmer declined to be interviewed for this study. Following the introductory chapter of this study, a review of the literature concerned with the analysis of leadership and personality is undertaken. The powers of the Prime Minister in the New Zealand political system are examined as are the events surrounding the execution of New Zealand's anti-nuclear policy and the ANZUS dispute. This thesis then assesses the effect of Lange's personality on the dispute through an examination of situational factors, and a variety of aspects of his personality. This thesis finds that Lange's personality was instrumental in determining the course of events in the ANZUS crisis. Furthermore, this study concludes that Hermann's theoretical framework is a useful tool in determining the effect of a leader's personality on a particular foreign policy outcome.