This project explores how growing Sino-US tensions have led to a contest over the regional security architecture governing the Asia-Pacific. ASEAN, still officially the default leader of regional formats such as the East Asia Summit, ASEAN Regional Forum, and ASEAN Defence Ministers Meeting Plus, has been increasingly sidestepped. The great power competitors have grown disinclined to dilute their agendas through ASEAN’s consensus mechanism and sought to formulate their own vehicles, such as the Quad Plus and Belt and Road Conference. Yet supplanting ASEAN structures requires legitimation, not only for the legitimacy of the alternative format, but also to attract support and buy-in, especially from ASEAN member states. Using Lenz and Söderbaum’s three analytical perspectives to classify US-led, China-led, and ASEAN-led initiatives in the region, the project investigates how discursive, behavioural, and isomorphic legitimation strategies have developed respectively for each actor.