In mid-1940, Nazi Germany rapidly defeated the French Third Republic, and the colonial administration of French Indochina passed to the French State. Many concessions were granted to the Empire of Japan, such as the use of ports, airfields, and railroads. Japanese troops first entered parts of Indochina in September 1940, and by July 1941 Japan had extended its control over the whole of French Indochina. The United States, concerned by Japanese expansion, started putting embargoes on exports of steel and oil to Japan from July 1940. The desire to escape these embargoes and to become self-sufficient in resources ultimately contributed to Japan's decision to attack on December 7, 1941, the British Empire and simultaneously the USA. This led to the USA declaring war against Japan on December 8, 1941. The US then joined the side of the British Empire, at war with Germany since 1939, and its existing allies in the fight against the Axis powers.

The Empire of Vietnam was a short-lived puppet state of Imperial Japan governing the former French protectorates of Annam and Tonkin between March 11 and August 25, 1945. At the end of its rule, the empire also successfully reclaimed Cochinchina as part of Vietnam.