Phibun exploited the Fall of France in June 1940 and the Japanese invasion of French Indochina in September 1940 to advance Thai interests in French Indochina following a border dispute with France. Phibun believed Thailand could recover territories ceded to France by King Rama V because the French would avoid armed confrontation or offer serious resistance. Thailand fought against Vichy France over the disputed areas from October 1940 to May 1941. The technologically and numerically superior Thai force invaded French Indochina and attacked military targets in major cities. Despite Thai successes, the French tactical victory at the Battle of Ko Chang prompted intervention from the Japanese, who mediated an armistice where the French were forced to cede the disputed territories to Thailand.