After the Fall of France in 1940, Cambodia and the rest of French Indochina were ruled by the Axis-puppet Vichy France government and despite an invasion of French Indochina, Japan allowed French colonial officials to remain in their colonies under Japanese supervision. In December 1940, the French-Thai War erupted and despite French resistance against the Japanese backed Thai forces, Japan compelled French authorities to cede Battambang, Sisophon, Siem Reap (excluding Siem Reap town) and Preah Vihear provinces to Thailand.^[82]

The subject of European colonies in Asia was among those discussed during the war by the Allied leaders of the Big Three , Franklin D. Roosevelt, Stalin, and Churchill at the three summit meetings - Cairo Conference, Tehran Conference and Yalta Conference. In regards to non-British colonies in Asia, Roosevelt and Stalin had decided in Tehran that the French and the Dutch would not return to Asia after the war. Roosevelt's untimely death before the war's end, was followed by developments very different from what Roosevelt had envisaged. The British backed the return of French and Dutch rule in Asia and organised dispatches of Indian soldiers under British command for this purpose.^[83]

In an effort to enlist local support in the final months of the war, the Japanese dissolved the French colonial administration on 9 March 1945, and urged Cambodia to declare its independence within the Greater East Asia Co-Prosperity Sphere. Four days later, King Sihanouk decreed an independent Kampuchea (the original Khmer pronunciation of Cambodia). On 15 August 1945, the day Japan surrendered, a new government was established with Son Ngoc Thanh acting as prime minister. When an Allied force occupied Phnom Penh in October, Thanh was arrested for collaboration with the Japanese and was sent into exile in France to remain under house arrest.