This book examines the creation and collapse of the first power-sharing administration in Northern Ireland, and British government policy during the period 1972-75. It also analyses the relationship between the British and Irish states during the 1972-75 period. Drawing on recently released archival sources, this book sheds new light on the events of this turbulent period, and questions many core assumptions about the political dynamics of the time. In particular, it challenges existing interpretations of the relationship between the Irish government and the representatives of constitutional nationalism in Northern Ireland and security co-operation between the British and Irish states. It revises the widely held view that the Irish government sought to use the Sunningdale Agreement's proposed Council of Ireland as a means of securing future Irish unity. It also examines the socio-economic problems which Northern Ireland faced at this time, and the attempts of the region's politicians to solve them, demonstrating the remarkable similarities between nationalist and unionist parties with regard to non-constitutional matters. It also challenges the view that the power-sharing experiment can be seen as a 'lost peace process', due to the levels of violence between 1972 and 1975. However, it demonstrates that the Sunningdale package, and the policies established during this period, provided the basic template for the current settlement in the region. It will thus be essential reading for students and scholars of modern British and Irish politics and the Northern Ireland conflict, and those interested in the politics of conflict resolution.