This thesis examines how the P3 states (France, the UK and USA) practically resolve tensions between their liberal preferences, practices of intervention, and the humanitarian solidarism of contemporary international society typified by R2P. It argues that where they behave as liberal vanguards, their practice threatens the sustainability of the solidarist international society typified by R2P. Using the cases of Côte d’Ivoire, Libya and Syria (2010-2012), it is argued that the P3 states either discursively advocated or attempted in practice liberal intervention which sought a change of regime or brought about actual regime change, contrary to the R2P normative framework which legitimates humanitarian intervention on a case-by case basis, subject to existing understandings of sovereignty, non-intervention, non-interference, limits on the use of force and multilateralism. In doing so, the P3 states’ approach to international legitimacy and attitude to international consensus was such that they behaved in practice as liberal vanguards: denying the gap between their practices and international norms; being unwilling to compromise over their goals; fostering and referencing alternative constituencies of legitimation other than the UN Security Council. These practices threaten the sustainability of the form of solidarist international society typified by R2P because they: confuse and potentially erode in practice the consensus understandings of the R2P normative framework; foster international discord among the great powers and between them and international society; mean that the Great Powers claim or even confer international legitimacy for themselves rather than having it conferred by the authoritative constituency of the UNSC; suggest that these powerful states do not believe themselves bound by the consensus principles that institute and constitute the society.

Patterns of domestic political contestation in international affairs often see the centre aligned against both the left and the right of the ideological spectrum. This is observable in a range of issues, from democracy promotion, intervention, international law, European integration, free trade, globalization and the creation of international regimes. Why centre-periphery ideological competition occurs is an interesting puzzle, given the challenge it offers to the idea that partisanship is an inherently left-right phenomenon. Yet the role of the political centre in foreign policy has not been subjected to systematic analysis. This thesis studies the nature and effects of the foreign policy position of the political centre. It argues that the centre is distinguished from left and right by its embrace of distinct elements of liberal ideology. The liberal view of international politics differs in thee important respects from its socialist and conservative competitors: It is particular, rather than pluralist, when it comes to questions of sovereignty and international legitimacy; it views interdependence, rather than independence, as a natural and desirable condition of the international; and it views deterrence, rather than diplomacy, as the best means of achieving security. To test the validity of this thesis I discuss the role of ideology in explaining variation in relations between four Euratlantic states (Britain, France, West Germany and Canada) and the United States during the Cold War. This is a hard case given the intensity of global threat at the time. The thesis tests the claim that the strength of Euratlantic-American relations is a function of the relative influence of the political centre at the time. To do this it outlines a mixed-methods research design that combines in-depth case studies with a quantitative analysis of Euratlantic-US relations. The results from both elements confirm the validity of the theoretical proposition.