One period of Vietnamese history that I find fascinating is World War II. During the War, Vietnam was occupied by the Japanese, but for most of the war the Japanese left the French in power.
France, however, had been occupied by the Germans, so the French colonial officials in Indochina during the war were part of a collaborating government known as Vichy France.
Vichy France was led by Philippe Pétain, a military man and authoritarian. He sent Admiral Jean Decoux to Indochina to promote his authoritarian agenda and to try to keep the Japanese from gaining influence among the Vietnamese.