<P> U.S. submarines participated in the majority of naval battles in the Pacific theatre, but the submarines were most decisive in their blockade of Japan, for which Japan was dependent on its sea transport to provide resources for its war effort . </P> <P> On the afternoon of 7 December 1941, six hours after the Japanese attack, U.S. naval commanders in the Pacific were ordered by the U.S. Navy Chief of Staff to "execute unrestricted air and submarine warfare against Japan". This order authorized all U.S. submarines in the Pacific to attack and sink any warship, commercial vessel, or civilian passenger ship flying the Japanese flag, without warning . The Pacific Fleet and the Asiatic Fleet Submarine Force immediately went into action to counter the Japanese offensive across the Pacific, such as in the Philippines, Indochina, Dutch East Indies and Malaya . The U.S. Navy submarine force was small; less than 2% . On 7 December 1941, the U.S. Navy had 55 fleet and 18 - medium - sized submarines (S - boats) in the Pacific, 38 submarines elsewhere, and 73 under construction . By war's end, the U.S. would complete 228 submarines . </P> <P> U.S. Navy submarines were often used for surveillance . This included reconnaissance U.S. submarines landed and supplied guerillas in Japanese occupied territory and carrying in commandos for missions such as the Makin Island raid, they also rescued crews of aircraft which had been forced down over the ocean . </P> <P> As a result of several key improvements in strategy and tactics, from 1943, Allied submarines waged a more effective campaign against Japanese merchant shipping and the IJN, in effect strangling the Japanese Empire of resources . By the end of the war in August, 1945, U.S. Navy submarines sank around 1300 Japanese merchant ships, as well as roughly 200 warships . Only 42 U.S. submarines were sunk in the Pacific, but 3,500 (22%) submariners were killed, the highest casualty rate of any American force in World War II . The force destroyed over half of all Japanese merchant ships, totaling well over five million tons of shipping . </P>

Where did the united states win its first major victories in world war ii