ABSTRACT This article uses the rise of the doctrine of ‘national security’ in US statecraft and strategy to re-examine Brian McKercher’s work on the ‘transition of power’ from Britain to the United States. World War II marked the great watershed in American foreign relations, in particular the ability, and more specifically the willingness, of Americans to project power beyond the western hemisphere. Not coincidentally, the war also marked another crucial turning point in American war and diplomacy: the transition from concepts like ‘national self-defence’ and ‘the national interest’ to a strategic doctrine that blended the blunt necessity of defence with the capacious ambition of interests. This new doctrine of ‘national security’ conceived of self-defence in a new way, not just as the protection of territorial sovereignty but as the protection of values, morals, and political norms. This much more capacious idea of defence was designed to be the bastion of a new liberal order to safeguard Western civilisation. In this light, American internationalists conceived of US power as the rightful heir to the mandate of the British Empire, albeit in ways that privileged command of the global commons over direct control of foreign territory. This article investigates the construction of ‘national security’ and the nature of Anglo-American empire in a new light.