This chapter will trace the principles of Thai diplomacy and the long history of Thailand’s external relations with the aim of better understanding the country’s foreign policy and its impact on both the regional and international political order. While there will be some discussion of ancient kingdoms, the focus will be on the 19th and 20th centuries, during which the country was exposed to the complexities of a truly international order. The hallmarks of Thai foreign policy – flexibility, pragmatism, and opportunism – will be discussed through an examination of major historical eras in Thailand’s international relations: the late 19th- and early 20th-century European imperial rivalry in Southeast Asia; the influence of the United States and ideas of democracy in the interwar period; the rise of ultra-nationalism in Japan and the road to the Second World War; a close partnership with the United States in the Cold War; and the struggle to maintain independence in the face of communist victories in Indochina and the withdrawal of American power. This will hopefully set the stage for more in-depth examinations of Thailand’s relationship with the Great Powers, regional organisations like ASEAN, and the array of both traditional and non-traditional security issues it confronts today.