Task: Question Generation
Input (English): While Japan had a large number of submarines, they did not make a significant impact on the war. In 1942, the Japanese fleet subs performed well, knocking out or damaging many Allied warships. However, Imperial Japanese Navy (and pre-war U.S.) doctrine stipulated that only fleet battles, not guerre de course (commerce raiding) could win naval campaigns. So, while the US had an unusually long supply line between its west coast and frontline areas, leaving it vulnerable to submarine attack, Japan used its submarines primarily for long-range reconnaissance and only occasionally attacked U.S. supply lines. The Japanese submarine offensive against Australia in 1942 and 1943 also achieved little.
Output (English): What was the primary use of Japanese submarines?