Quote:
U.S. Submarine Warfare Directives The Chief of Naval Operations issued a directive on 7 December 1941 to "Execute unrestricted air and submarine warfare against Japan". The major mission assigned our submarines was the anti-ship (torpedo attack) mission. Early in the war Japanese capital ships were assigned as primary targets. Later, the priority was placed on merchant ships in order to cut off the Japanese supply of critical war materials, fuel, and food to her wide-spread ocean empire and to the home islands.
On 13 April 1944, our submarines were instructed to give priority to fleet destroyers. The purpose was to reduce the major defensive strength of Japanese combatant groups and high priority merchant shipping.
Later still, the highest priority was placed on tankers. The objective was to cut off fuel to the fleet and the Japanese home islands.
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The above quote is from......
https://archive.hnsa.org/doc/subsinpacific.htm
Hope this helps you out Bubblehead1980.
