Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Politics, Philosophy and Legal Studies, College of Arts, Social Sciences and Commerce, La Trobe University, Bundoora.This thesis uses the notion of strategic culture to examine Thailand’s approach to its external security. It surveys Thai history from the late nineteenth century to: (i) identify possible elements of a Thai strategic culture; (ii) understand how that culture is sustained and how it may influence decision-making and; (iii) consider correlations between Thai strategic behaviour and that culture. The thesis finds that at the politico-military level, the Thai state developed a distinctive strategic culture during the late nineteenth and early twentieth centuries. It comprises politico-military narratives drawn from Thai royalist-nationalist history and corresponding to Thailand’s efforts to meet two security challenges: finding security in a world dominated by powerful European states and constructing a viable state while limiting pretexts for external intervention. The narratives impart a high value to diplomacy and national unity, together with a belief in the necessity of military force as a final deterrent. The thesis also analyses Thai strategic culture at the level of Thai military organisational culture. This organisational culture has three characteristics: royalism, factionalism and army dominance. The thesis tests this model of strategic culture in three case studies: (i) Thailand’s response to Vietnam’s invasion and occupation of Cambodia 1979-1989, (ii) Thai defence budgeting and procurement, and (iii) the Thai-Cambodia temple dispute 2008-2011. The case studies find aspects of Thai strategic-level behaviour correlate with the imperatives of Thai politico-military narratives. The case studies also show that an analytically eclectic approach, comprising Thai military organisational culture, Thai civil-military relations and generic organisational factors can help explain outcomes at the operational level. These outcomes include weak command and fragmented military doctrine leading to low proficiency in state-on-state military operations. The thesis argues that Thailand’s strategic culture acting alongside the Thai political system has contributed to strategic success amidst operational inefficiency.