Indochina was a French colony east of Thailand and south of China with the South China Sea to the east, so named due its position halfway between India and China. In the mid 20th century, the colony birthed three countries: Cambodia, Laos, and Vietnam.
During World War II, Indochina was owned by Vichy France. Japan was given the rights to base troops in Indochina by the Vichy French. This was also noted that if France denied Japan, the Japanese would have gone in anyway. This way, France maintained a ghostly sort of sovereignty over the area.
Following France's defeat in the Great War, the Empire of Japan "persuaded" France to hand over the former French Indochina for a suitable amount of compensation which came as a respite for the battered French economy.
When World War II began in 1938, French forces in the colony were left isolated as France itself came under attack from Germany, and almost lost the capital.
On January 12, 1941, the Empire of Japan attacked Indochina, and quickly conquered it.