Japan justified its occupation of South-East Asian territories to the international community by framing its military actions as efforts to liberate Asia from Western colonialism and establish the Greater East Asia Co-Prosperity Sphere. Japanese propaganda portrayed their expansion as a crusade against Western imperialism, aimed at bringing about an Asia for Asians. Japan also emphasised the concept of Pan-Asianism, arguing that its role was to lead Asia in a united front against Western domination. This rhetoric, however, thinly veiled Japan's own imperial ambitions and the harsh realities of its occupation, which involved significant exploitation of the conquered territories' resources and populations.

Japan's long-term strategic plan for South-East Asia, had they won the war, was centred around the concept of the Greater East Asia Co-Prosperity Sphere. This plan entailed the integration of the region into a Japan-led bloc designed to be free from Western influence, with a network of puppet and client states providing resources and political support for Japanese hegemony. Economically, Japan aimed to create a self-sufficient bloc that would consolidate its access to the region's resources. Politically, it intended to establish a new order based on pan-Asian unity, albeit under Japanese leadership, which would also serve as a buffer against any future Western attempts to regain control.

The economic consequences of Japanese victories for the conquered South-East Asian territories were devastating. Japan’s policy of economic exploitation led to the redirection of agricultural and industrial production to meet Japanese military needs. This included the seizure of commodities like rice, which caused widespread food shortages and famine in regions like Vietnam, where the Great Vietnamese Famine of 1944-45 occurred. The conscription of local labour for military projects and the extraction of natural resources, such as oil and rubber, decimated local economies and left long-term scars on the economic landscape of South-East Asia, the effects of which persisted well into the post-war period.

Japan's initial victories severely undermined the credibility and authority of colonial powers in South-East Asia. The swift capitulation of European colonies exposed their vulnerability and emboldened independence movements across the region. Colonial powers such as the Netherlands, Britain, and France were forced to confront the reality that their reign in the region was no longer tenable in the post-war order. This realisation, coupled with the devastation of their own economies by the war and the inexorable push for decolonisation globally, led to the eventual withdrawal of colonial powers and the rise of independent nation-states in South-East Asia.

The Thai government, under the premiership of Field Marshal Plaek Phibunsongkhram, initially declared neutrality. However, after the Japanese forces advanced into Thai territory on their way to Malaya and Singapore, Thailand was compelled to ally with Japan. This alliance was formalised with the signing of a military alliance on 21 December 1941, and subsequently, Thailand declared war on the United States and the UK. The Thai government's decision was influenced by realpolitik, as it sought to reclaim territories lost to the British and French and further national interests amidst the changing dynamics of power in South-East Asia.