Since the mid-1990s, ASEAN-led multilateral mechanisms have become important venues where Washington and Beijing negotiate regional affairs with each other, with Southeast Asian states, and with others. China’s embrace of ASEAN multilateralism is a move to maintain a stable external environment and to bolster its growing influence in the region. China’s proactive engagement and increasing influence prompted the US to reconsider the utilities of multilateralism in its bilateralism-centered regional policy and to participate in ASEAN multilateralism. Moves by these great powers toward ASEAN-led multilateralism necessitated accommodations to the ASEAN approach to and agenda for regional security, namely conceding some of their own power of agenda setting. This implies that Southeast Asian states have obtained more political autonomy in architecting preferred regional development. US–China competition over regional alliance networks has also given ASEAN states a relatively flexible strategic environment that allows them to hedge against Beijing and Washington to mitigate uncertainty and increase their international political autonomy. The case studies of the ARF, the Philippines, and Vietnam demonstrate this tendency in Southeast Asia, supporting the theoretical argument that small states can obtain greater international political and strategic autonomy during great power competition in the power transition context.