The submarine crews underwent several training exercises in 1938 and 1939. In November 1940, following a number of border skirmishes, the undeclared Franco-Thai War over disputed border areas began when the Royal Thai Air Force made air raids on military bases in French Indochina. The navy was mobilised to protect Thailand's territorial waters, and the submarines conducted reconnaissance in the Gulf of Thailand. However, they were unable to prevent a surprise French naval raid, which resulted in heavy Thai naval losses at the Battle of Ko Chang on 17 January 1941. Following the battle, the submarines were sent to patrol the vicinity of Ream Naval Base in present-day Cambodia, but no further naval clashes took place and Japan soon negotiated an end to the war.