Allied submarines did not adopt a defensive posture and wait for the enemy to attack. Within hours after the Pearl Harbor attack, in retribution against Japan, Roosevelt promulgated a new doctrine: unrestricted submarine warfare against Japan. This meant sinking any warship, commercial vessel, or passenger ship in Axis-controlled waters, without warning and without help to survivors.[nb 13] At the outbreak of the war in the Pacific the Dutch Admiral in charge of the naval defense of the East Indies, Conrad Helfrich, gave instructions to wage war aggressively. His small force of submarines sank more Japanese ships in the first weeks of the war than the entire British and US navies together, an exploit which earned him the nickname "Ship-a-day Helfrich". The Dutch force were in fact the first to sink an enemy warship; On 24 December 1941, HNLMS K XVI torpedoed and sank the Japanese destroyer Sagiri.
When was the first Japanese warship sunk by a submarine?
24 December 1941