This book provides an original account of how the Association of Southeast Asian Nations' (ASEAN) has responded to potential violations of its members' sovereignty over the past 50 years. Drawing on rarely analysed documents – including declassified government cables and reports, WikiLeaks cables, and state correspondence to the United Nations — and examining in detail four key cases, it provides a compelling account of why states act as they do in the face of potential sovereignty violation.  By showing how state agency, regional interests and external powers are all factors in the ability to resist sovereignty violation, the author develops the 'vanguard state theory', which, it is argued, has universal applicability and explanatory power.