Since the end of the absolute monarchy in 1932, Thailand has spent more time under a military regimes than under civilian rule. The military and its conservative allies have tended to define what are the threats to national security. Despite a change in socio-political circumstances in the postcounterinsurgency period, new threats have been defined which provide the rationale and legitimacy for the military, especially the army, to expand its socio-political role. In contrast, civilian governments have paid little attention to this matter. This chapter examines the development of the power of the military—the army in particular—over internal security affairs.

The chapter contains two sections. The first provides the basic understanding of the Thai state's counterinsurgency operations against the Communist Party of Thailand (CPT) since the 1950s. It begins with a brief description of the origin of the struggle against the CPT and how this struggle defined the Thai state's perception, strategy and operations on internal security affairs. The cooperation among the military, the monarchy and the United States is discussed. Then, I trace the emergence of the two key concepts of Thailand's counterinsurgency operations: the concept of political offensive (kanmueang nam kanthahan) and the concept of development for security (yutthaphatthana). These two concepts were the basis for the expansion of the military role in the socio-economic and political development spheres during the counterinsurgency period, and were sustained with modifications in the post-counterinsurgency period. I demonstrate that ISOC was not the only agency involved in internal security operations as commonly understood. All branches of the Thai armed forces, including the army, navy, air force and the Supreme Command, have been extensively involved in the country's socio-political affairs at least since the early 1980s when the communist threat was in decline. Internal security has been the leading mission of the Thai armed forces.

In the post-counterinsurgency period the threats to national security no longer involved combat and hence should have been the responsibility of civilian agencies, yet the military continued to claim that managing these threats was its main mission. The second section shows how the military secured legal and political legitimacy to continue its wider sociopolitical role from the counterinsurgency period until the present day.