In 1940, during the early days of WW II, Japan began demanding the cooperation of French colonial authorities in Vietnam as part of the Japanese war effort against China. By the summer of 1941, Japanese military forces had occupied all of Vietnam. Although this led directly to the imposition of Allied sanctions against Japan and the outbreak of a wider war in the Pacific, within Vietnam itself the collaborationist Vichy French regime that was in place by that time did not resist. Despite Japanese occupation, routine French colonial administration of Vietnam continued throughout most of WW II, until March 1945, when the Japanese finally seized power directly and disarmed and interned the French. The following day, the Japanese named the last Nguyen Dynasty ruler, Bao Dai (1913–1997; emperor, 1926–1945; head of state, 1949–1955), emperor of a nominally independent Vietnam, within the Japanese-controlled Greater East Asia Co-Prosperity Sphere.
Meanwhile, the Japanese occupation of Vietnam during WW II had created an opportunity for the Vietnamese communists to call for a united front of all patriotic Vietnamese to resist Japan. After spending thirty years in exile, some time in late 1940 or early 1941 the communist leader Nguyen Sinh Cung returned to Vietnam (where he finally actually assumed his most famous alias, “Ho Chi Minh”). There, at a meeting near the Chinese border in May 1941, a front organization (deliberately intended to downplay the communist leadership role) was formed – called the League for the Independence of Vietnam, but commonly known as the Viet Minh – and it began to conduct guerrilla warfare against the Japanese.
The French Withdrawal, and America's War
The Japanese surrender at the end of WW II, in August 1945, left Vietnam in chaos. Northern Vietnam at the time of surrender was only just beginning to recover from a catastrophic famine that had claimed the lives of one or two million people earlier that spring. Ho Chi Minh and the Viet Minh appeared to be virtually the only forces on the ground that were opposing the Japanese and attempting to deal with the famine – by, for example, leading attacks on warehouses where rice had been diverted and stored for Japanese use.