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.

French Indochina, officially known as the Indochinese Union and after 1947 as the Indochinese Federation, was a grouping of French colonial territories in Mainland Southeast Asia until its end in 1954. It comprised Cambodia, Laos, the Chinese territory of Guangzhouwan, and the Vietnamese regions of Tonkin in the north, Annam in the centre, and Cochinchina in the south. The capital for most of its history (1902–1945) was Hanoi; Saigon was the capital from 1887 to 1902 and again from 1945 to 1954.