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 under the nominal rule of the Vichy government of France, which gave Japan permission to base troops in Indochina. Admittedly, if France had denied this to Japan, the Japanese would have gone into Indochina anyway. This way, France maintained a ghostly sort of sovereignty over the area.
FrenchIndochina was restive in 1914, and the outbreak of the Great War exacerbated that restiveness. In September, 1914, Karl von Müller, captain of the GermanleviathanEmden, ordered a successful attack on French oil stores outside of Saigon. The attack hurt France's war machine, but Müller also wanted to further motivate the people of Indochina to rebel.
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.