This thesis looks at the evolution of the interaction between the Ulster Unionist Party and the US government during the peace process that led to the signing of the Good Friday Agreement and its implementation. It uses mediation in ethnic conflict resolution as a theoretical framework as it defines the role that the US played in the peace process, presenting itself as an honest broker. This thesis contributes, firstly, to the gap in the literature on the unionist perception of the US involvement and provides new insight into the peace process. Secondly, it contributes to the wider debate in international mediation using Ulster Unionists-US relationship as a case study, in order to define to what extent leverage guarantees a long-term success. If the US mediation in Northern Ireland has been fruitful in that it contributed to the signing of the agreement, and somehow established links with the most hostile actors, its long-term success is still questionable. It actually operated in relation to the Unionists who had not clearly perceived any kind of hurting stalemate and whose internal division caused a serious threat to peace. The monitoring of the implementation of the agreement revealed that the majority of the Unionist community remained very hostile towards it. Thus, despite David Trimble’s pragmatic approach to reform Unionist strategy, he did not convince his electorate of the quality of the agreement. The primary cause for this is the lack of internal motivation within Unionism to support the agreement. Therefore, this study teaches us that even if leverage is an essential parameter in mediation, it cannot overcome the lack of internal driving force.