Allied submarines in the Pacific War Allied submarine s were used extensively during the Pacific War and were a key contributor to the defeat of the Empire of Japan . During the war , submarines of the United States Navy were responsible for 55 % of Japan ' s merchant marine losses ; other Allied navies added to the toll . The war against shipping was the single most decisive factor in the collapse of the Japanese economy . Allied submarines also conducted reconnaissance patrols , landed special forces and guerrilla troops and performed search and rescue tasks . The majority of the submarines involved were from the U . S . Navy , with the British Royal Navy and the Royal Netherlands Navy contributing smaller numbers of boats . The Allied submarine campaign is one of the least - publicized feats in military history , due in large part to the efforts of Allied governments to ensure their own submarines ' actions were not reported in the media . The U . S . Navy adopted an official policy of unrestricted submarine warfare , and it appears the policy was executed without the knowledge or prior consent of the government . The London Naval Treaty , to which the U . S . was signatory , required submarines to abide by prize rules ( commonly known as " cruiser rules " ) . It did not prohibit arming merchantmen , but arming them , or having them report contact with submarines ( or raiders ) , made them " de facto " naval auxiliaries and removed the protection of the cruiser rules . This made restrictions on submarines effectively moot . While the Allies were conducting a massive propaganda campaign against the actions of German U - boat s , their own submarines were operating exactly as the Germans did . Strategic implications . Throughout the war , Japan was dependent on sea transport to provide adequate resources , including food , to the home islands and supply its military at garrisons across the Pacific . Before the war , Japan estimated the nation required of shipping to maintain the domestic economy and military during a major war , which was considerably less than the of shipping in the Japanese merchant fleet and of smaller craft at the time of the attack on Pearl Harbor . Despite the awareness shipping was vital , the Japanese military accorded anti - submarine warfare a low priority and allocated few warships and aircraft to protecting merchant shipping For instance , their destroyers had deficiencies in sonar and radar compared to equivalents of other navies , despite their impressive night fighting capabilities , even though these warships formed the bulk of convoy protection . Moreover , Japanese Navy doctrine in relation to commerce defense was derisively bad . The Japanese also seriously underestimated the threat from Allied submarines , whose ineffectiveness in the early part of the war reinforced Japanese overconfidence . The size and effectiveness of the Allied submarine force increased greatly during the Pacific War . At the start of the war , a high proportion of the submarines deployed against the Japanese were obsolete , and U . S . boats were hampered by defects in their primary weapon , the Mark 14 torpedo , as well as by poor training ( an excessive reliance on sonar , due to an undue fear of destroyer s ' sonar and aircraft ) , insufficiently aggressive skippers , poor dispositions ( scattered on close surveillance of Japan ' s major bases ) , and divided command ( which kept submarines out of one of the best hunting areas , the Luzon Strait , for fear of fratricide ) . Growing numbers of modern submarines became available from 1942 onwards . The efforts of Admiral Charles A . Lockwood were crucial for the rectification of the Mark 14 ' s problems ( which were nevertheless not resolved until September 1943 ) , and for the selection of more aggressive submarine skippers . As a result of all of these developments , assisted by signals intelligence ( breaking the " " maru " code " in January 1943 , after a gaffe by U . S . Customs prewar had caused Japan to change it ) , U . S . submarines inflicted devastating losses on Japanese merchant shipping in 1943 and 1944 . In conjunction with attacks by aircraft , including aerial minelaying , U . S . submarines had effectively destroyed the Japanese merchant fleet by January 1945 . Poor torpedoes claimed at least two U . S . submarines out of 42 lost on patrol . Prelude . The U . S . had the largest and most powerful submarine force of all the Allied countries in the Pacific at the outbreak of war . Pre - war U . S . Navy doctrine — like that of all major navies — specified the main role of submarines was to support the surface fleet by conducting reconnaissance and attacking large enemy warships . Merchant ships were regarded as secondary targets , and the circumstances in which they could be attacked were greatly limited by prize rules set out in the London Naval Treaty , to which the U . S . was a signatory . In order to meet their role with the surface fleet the U . S . Navy built large submarines which boasted long range , a relatively fast cruising speed and a heavy armament of torpedo s . U . S . submarine habitability was greatly enhanced by air conditioning ( which German U - boat s , for instance , lacked ) and water distilleries , and were more comfortable , hence better suited to long patrols in the tropics , than those of the other major powers . The submarines ' commanders and crewmen were considered an elite and enjoyed a strong " esprit " . On 7 December 1941 , the USN 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 . ) While Britain stationed a force of submarines in the Far East prior to the outbreak of war , none were available in December 1941 . The British had 15 modern submarines in the Far East in September 1939 . These submarines formed part of the China Station and were organised into the 4th Flotilla . Although the number of British submarines in the Far East increased in early 1940 when the 8th Flotilla arrived at Ceylon , both flotilla s and all their submarines were withdrawn in mid - 1940 to reinforce the Mediterranean Fleet . The Netherlands also maintained a submarine force in the Far East in order to protect the Netherlands East Indies ( NEI ) . In December 1941 , this force comprised 15 boats based at Surabaya , most of which were obsolete . Countering the Japanese offensive . In a break with pre - war doctrine ( which , like Japan ' s , had presumed a rush across the Pacific and a " decisive battle " between battleship s ) with the London Naval Treaty , and with long - standing U . S . defense of freedom of the seas , 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 " on the afternoon of 7 December 1941 , a mere six hours after the Japanese attack . 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 . Hart — on his own initiative ( but knowing Stark intended to do so ) — issued the same order at 03 : 45 Manila time ( 09 : 15 in Hawaii , 14 : 45 in DC ) . The Pacific Fleet Submarine Force had emerged unscathed from the attack on Pearl Harbor and departed on the fleet ' s first offensive war patrol on 11 December . The Asiatic Fleet ' s 27 submarines ( including more fleet boats than at Pearl Harbor ) also went into action on the first day of the war , beginning war patrols in the waters around the Philippines and Indochina . Thanks to inadequate prewar planning , which made no provision for defensive minelaying , nor placing submarines on station around the Philippines , nor off enemy harbors , the Asiatic Fleet ' s efforts to counter the Japanese invasion of the Philippines were unsuccessful and the Fleet ' s surviving submarines were forced to withdraw to Surabaya in the Dutch East Indies ( DEI ) . British , Dutch , and U . S . submarines took part in the unsuccessful defense of British Malaya and the DEI in late 1941 and early 1942 . In December 1941 , five Dutch submarines attacked the Japanese invasion fleet off Malaya . These submarines sank two Japanese merchant ships and damaged four others , but three of the attackers were sunk . The two surviving Dutch submarines were withdrawn to defend the DEI where they were assisted by two British submarines , which had been transferred from the Mediterranean Fleet , and several U . S . boats . The U . S . Asiatic Fleet ' s submarine force left Surabaya for Fremantle in Australia on 1 March . ( They would remain in Australia , on the most hazardous and unproductive stations for U . S . submarines , for the duration . ) By this date , the Asiatic Fleet ' s 27 submarines had sunk only 12 Japanese ships for the loss of four U . S . boats . Following the fall of the DEI , only a handful of British and Dutch submarines were based in the Indian Ocean , and these had little impact on Japanese forces in the area . War of attrition . After the Battle of Coral Sea , the U . S . Navy detached eight submarines to finish off the damaged aircraft carrier , but she evaded all of them . At the Battle of Midway , although the attack on the battleship by had been unsuccessful , it drew the destroyer " Arashi " temporarily away from the main fleet to drop depth charges , and the destroyer ' s return was traced by ' s VB - 6 to the Japanese task force , where the dive bombers promptly sank four fleet carriers . Overall in 1942 , U . S . submarines had only managed to sink the heavy cruiser and the light cruiser . As a result of several key improvements the previous year , U . S . submarines inflicted tremendous losses to the heavy units of the Imperial Japanese Navy in 1944 . They destroyed the Japanese fleet carriers " Shōkaku " and in the Battle of the Philippine Sea , and sank or disabled three " Takao " - class cruisers at the start of the Battle of Leyte Gulf . Also sunk that year were the battleship ( being the only Japanese battleship lost to a submarine ) and the carrier " Shinano " , the latter being the largest vessel ever lost to submarine torpedoes . From 1943 , Allied submarines waged an increasingly effective campaign against Japanese merchant shipping and the IJN . By the end of the war in August 1945 , the Japanese merchant marine had less than a quarter of the tonnage it had in December 1941 . Overall , U . S . Navy submarines sank around 1300 Japanese merchant ships , as well as roughly 200 warships . Despite the need to maintain sea lanes for its empire , the Japanese never successfully developed a cost - effective destroyer escort better suited for convoy duties , while it also did not have the industrial - military complex to replace the losses of its heavily - armed destroyers , nor of its ill - protected merchantmen . By contrast , after the U . S . revealed the weaknesses of Japan ' s depth charges in 1943 , Japanese anti - submarine warfare grew in effectiveness , particularly after the debut of radar in the IJN . British and Dutch submarine operations . The British submarine force in the Far East was greatly expanded from August 1943 . The British Eastern Fleet was responsible for submarine operations in the Bay of Bengal , Strait of Malacca as far as Singapore , and the western coast of Sumatra to the Equator . Few large Japanese cargo ships operated in this area , and the British submarines ' main targets were small craft operating in inshore waters . The submarines were deployed to conduct reconnaissance , interdict Japanese supplies travelling to Burma , and attack U - boats operating from Penang . The Eastern Fleet ' s submarine force continued to expand during 1944 , and by October 1944 had sunk a cruiser , three submarines , six small naval vessels , of merchant ships , and nearly 100 small vessels . The British submarine force expanded its areas of operation in the last months of the war . In late 1944 , the Eighth Flotilla — with 11 British and Dutch submarines — was transferred to Fremantle and operated in the Java Sea and surrounding areas under the command of the U . S 7th Fleet . The Fourth Flotilla and the newly - formed Second Flotilla remained at Ceylon . By March 1945 , British boats had gained control of the Strait of Malacca , preventing any supplies from reaching the Japanese forces in Burma by sea . By this time , there were few large Japanese ships in the region , and the submarines mainly operated against small ships which they attacked with their deck guns . In April the Eighth Flotilla moved to Subic Bay in the Philippines and the Fourth Flotilla replaced it at Fremantle . At this time , there were 38 British and Dutch submarines in the theater , and an additional five boats on their way from Europe . The submarine torpedoed and sank the heavy cruiser in the Bangka Strait , taking down some 1 , 200 Japanese army troops . Three British submarines ( , , and ) were sunk by the Japanese during the war . Merchant shipping losses . As figures for the size of the Japanese merchant marine and its losses differ between different sources , it is impossible to provide a definitive accounting of the state of the merchant marine over the course of the war . The following tables show different assessments of Japanese losses and construction . Size of the Japanese merchant fleet during World War II ( all figures in tons ) Japanese merchant fleet losses during World War II ( all figures in tons , taken from JANAC ) One Japanese reference reports 15 , 518 civilian ships lost . JANAC reports 2 , 117 Japanese merchant ships lost with a total tonnage of and 611 IJN ships lost with a total tonnage of . Other duties . Allied submarines served in a range of other duties during the Pacific War . U . S . Navy submarines were often used for surveillance . This included taking photos of areas of interest ( such as potential beaches for amphibious landings ) , and reporting on the movements of IJN warships . U . S . submarines landed and supplied reconnaissance and guerrilla forces and played a role in sustaining the guerrilla movement in the Philippines , at the cost of their diversion from productive use in attacks on Japanese commerce . They also occasionally transported commando s , such as " Nautilus " and landing Marine Raiders for an abortive raid on Makin Atoll . From early 1944 U . S . submarines were also used to rescue the crews of aircraft which had been forced down over the ocean . By the end of the war , submarines had rescued 504 airmen ( including George H . W . Bush , who later became the 41st President of the United States ) . British and Dutch submarines also landed and supplied special forces troops , rescued airmen , and shelled shore installations on nine occasions . Britain also deployed a flotilla of midget submarine s to the Far East which were used to conduct sabotage raids . The Fourteenth Flotilla , which was equipped with six " XE " - class submarines , arrived in Australia in April 1945 but was almost disbanded in May as no suitable targets could be found . The Flotilla ' s fortunes improved in early June , however , when undersea telegraph lines in the South China Sea were identified as being worthwhile targets along with a heavy cruiser at Singapore . On 31 July , " XE4 " cut the submerged Singapore - Saigon telegraph cable near Cape St . Jacques in French Indochina and " XE5 " cut the Hong Kong - Saigon cable close to Lamma Island , Hong Kong . At the same time , " XE1 " and " XE3 " penetrated the Straits of Johor where they severely damaged the Japanese heavy cruiser with limpet mine s . Post - war . Allied actions in the Pacific are believed to have been a mitigating factor in reducing the sentence of " Großadmiral " Karl Dönitz following the Nuremberg Trials , who was accused of similar actions in the Battle of the Atlantic ; indeed , Admiral Nimitz provided Dönitz with a statement saying his boats behaved no differently .