Thailand's unique role in the history of Japan's World-War-II occupation of Southeast Asia reflected the fact that, prior to the war, it was the only independent state in a region dominated by European colonial powers. It had became the object of rivalry between Great Britain, the chief defender of the status quo, and an expansive Japan in the 1930s. This competition ended when Japan moved troops into Thailand on 8 December 1941 and contracted an alliance with the government of Field Marshal Phibun Songkhram. Subsequently, the three chief allies fighting against Japan – Britain, the USA, and China – all developed schemes to counter the Japanese presence and promote their own influence in Bangkok after the war. In order to put these events in context, it is necessary to look briefly at the prewar history of Thailand's internal politics and foreign relations.

From 1782 until 1932, the kings of the Chakri Dynasty ruled Thailand, then known as Siam, as absolute monarchs. Their accommodationist diplomacy, favorable geography, and a measure of good luck enabled Siam to survive the high age of European imperialism somewhat diminished in size, but with its independence relatively intact. The revered king who presided over most of this critical period, Chulalongkorn (Rama V, r. 1868–1910), had implemented a program of reforms that modernized the administrative structure, strengthened Bangkok's control over the provinces, and greatly increased the dominance of the royal family at the expense of the once-influential court nobles and regional princes.