Franco-Thai War: French forces inflict a decisive defeat over the Royal Thai Navy.
The Battle of Ko Chang took place on 17 January 1941 during the Franco-Thai War in which a flotilla of French warships attacked a smaller force of Thai vessels, including a coastal defence ship. The battle resulted in a tactical victory by the French Navy over the Royal Thai Navy although the strategic result is disputed. The Japanese intervened diplomatically and mediated a ceasefire.
In the end, two Thai ships were sunk and one was heavily damaged. Within a month of the engagement, the French and the Thais negotiated a peace which ended the war.
The Franco-Thai War (October 1940 – January 28, 1941, Thai: กรณีพิพาทอินโดจีน, romanized: Krṇī phiphāth xindocīn; French: Guerre franco-thaïlandaise) was fought between Thailand and Vichy France over certain areas of French Indochina.
Negotiations with France shortly before World War II had shown that the French government was willing to make appropriate changes in the boundaries between Thailand and French Indochina, but only slightly. Following the Fall of France in 1940, Major-General Plaek Pibulsonggram (popularly known as "Phibun"), the prime minister of Thailand, decided that France's defeat gave the Thais an even better chance to regain the vassal state territories that were ceded to France during King Chulalongkorn's reign.
The German military occupation of Metropolitan France rendered France's hold on its overseas possessions, including French Indochina, tenuous. The colonial administration was now cut off from outside help and outside supplies. After the Japanese invasion of French Indochina in September 1940, the French were forced to allow Japan to set up military bases. This seemingly subservient behavior lulled the Phibun regime into believing that France would not seriously resist a military confrontation with Thailand.