Imperial Japanese Army troops bicycling through Saigon, c1942.

The Japanese Invasion of French Indochina, also known as the Vietnam Expedition, was a move by the Empire of Japan in September 1940, during the Second Sino-Japanese War, to prevent China from importing arms and fuel through French Indochina, via the Sino-Vietnamese Railway from the port of Haiphong through Hanoi to Kunming in Yunnan. Japan occupied northern Indochina, which tightened the blockade of China, and made continuation of the drawn out Battle of South Guangxi unnecessary. In March 1945, Japan launched the Second French Indochina Campaign and ousted the Vichy French and formally installed Emperor Bao Dai in the short-lived Empire of Vietnam. In August 1945, Japanese forces surrendered in Indochina at the end of World War II. (Photo by: Pictures From History/Universal Images Group via Getty Images)