How has the United States sought to shape its unipolar moment since the end of the Cold War? Has Washington’s liberal hegemonic leadership proved resilient or are we witnessing a shift towards an imperialist approach to international cooperation marked by a reliance on force and the imposition of policy preferences? This dissertation seeks answers from two angles. First, it argues that the way in which the US government has been dealing with the nuclear non-proliferation regime can teach us about the terms in which it may currently be seeking to shape the unipolar moment in part based on its nuclear supremacy. Through international institutions the powerful seek to shape the behaviour of state-actors in setting constitutive standards for being member of the community of states and norm or rules to regulate behaviour within it. Since the adoption of the National Strategy to Combat Weapons of Mass Destruction in December 2002, we are witnessing the legalization of the regime: The George W. Bush administration has since then adopted a series of policy initiatives formalizing and enforcing core norms of the nuclear Non-Proliferation Treaty based on counterproliferation policy tenets developed at US level since the launch of the Counterproliferation Initiative in 1993. In tracing the adoption and implementation phases of the Proliferation Security Initiative and of UN Security Council Resolution 1540, it scrutinizes Washington’s approach to international cooperation and its legitimation preferences to identify elements of hegemonic leadership and imperial domination. Second, it focuses on Washington’s evolving relations with France as Nuclear Weapon State and co-constitutive member of the transatlantic security community, Iran as ‘rogue’ state and India as de facto Nuclear Weapon State outside of the NPT realm. Ties between Washington and Paris are marked by an informal hierarchy, whilst Washington has sought to win India’s acceptance of its hegemonic leadership, and Iran has been ostracised from the international community on the grounds of its suspected nuclear military aspirations. Washington has arguably begun to draw the boundaries between ‘ins’ and ‘outs’ of its intersubjectively constituted hegemonic leadership on the basis of these state’s relation to the nuclear weapon.