US submarines, as well as some British and Dutch vessels, operating from bases at Cavite, in the Philippines (1941–42) Fremantle and Brisbane in Australia; Pearl Harbor; Trincomalee, Ceylon; Midway; and later Guam, played a major role in defeating Japan. This was the case even though submarines made up a small proportion of the Allied navies — less than two percent in the case of the US Navy. Submarines strangled Japan by sinking its merchant fleet, intercepting many troop transports, and cutting off nearly all the oil imports essential to weapons production and military operations. By early 1945 the oil tanks were dry. The Japanese military claimed its defenses sank 468 Allied subs. Only 42 US submarines were sunk in the Pacific, with 10 others going down in accidents or as the result of friendly fire.
The torpedoed Japanese destroyer, Yamakaze, as seen through the periscope of an American submarine, Nautilus, in June 1942. US submarines accounted for 56% of the Japanese merchantmen sunk; most of the rest were destroyed by mines or aircraft. US submariners also claimed 28% of Japanese warships destroyed. Furthermore, they played important reconnaissance roles, as at the battles of the Philippine Sea and Leyte Gulf (and, coincidentally, at Midway), when they gave accurate and timely warning of the approach of the Japanese fleet. Submarines also rescued hundreds of downed fliers. The Allied submarines did not adopt a defensive posture and wait for the enemy to attack. Within hours of Pearl Harbor, Roosevelt ordered a new doctrine into effect: 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. Allied submarine bases were well-protected by surface fleets and aircraft. 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 US) doctrine stipulated naval campaigns are won only by fleet battles, not guerre de course (commerce raiding). So, while the US had an unusually long supply line between its west coast and frontline areas, and was vulnerable to submarine attack, Japan's submarines were instead primarily used for long range reconnaissance and only occasionally attacked US supply lines. The Japanese submarine offensive against Australia in 1942 and 1943 also achieved little. As the war turned against Japan, IJN submarines were increasingly used to resupply strongholds which had been cut off, such as Truk and Rabaul. In addition, Japan honored its neutrality treaty with the Soviet Union and ignored US freighters shipping millions of tons of war supplies from San Francisco to Vladivostok much to the consternation of its German ally.
The Japanese submarine I-400. The Sen Toku I-400 class were the largest non-nuclear submarines ever constructed. Japanese submarines were not used to their full potential during the Pacific War. The US Navy, by contrast, relied on commerce raiding from the outset. However, the problem of Allied forces surrounded in the Philippines, during the early part of 1942, led to diversion of boats to "guerrilla submarine" missions. As well, basing in Australia placed boats under Japanese aerial threat while en route to patrol areas, inhibiting effectiveness, and Nimitz relied on submarines for close surveillance of enemy bases. Furthermore, the standard issue Mark 14 torpedo and its Mark VI exploder were both defective, problems not corrected until September 1943. Worst of all, before the war, an uninformed US Customs officer had seized a copy of the Japanese merchant marine code (called the "maru code" in the USN), not knowing Office of Naval Intelligence (ONI) had broken it; the Japanese government promptly changed it, and the new code was not broken again until 1943. Thus, it was not until 1944 the US Navy began to use its 150 submarines to maximum effect: effective shipboard radar was installed, commanders lacking in aggression were replaced, and faults in torpedoes were fixed. Japanese commerce protection was "shiftless beyond description," and convoys were poorly organised and defended compared to Allied ones, a product of flawed IJN doctrine and training — errors concealed by American faults as much as Japanese overconfidence. The number of U.S. submarines patrols (and sinkings) rose steeply: 350 patrols (180 ships sunk) in 1942, 350 (335) in 1943, and 520 (603) in 1944. By 1945, sinkings had decreased because so few targets dared to move on the high seas. In all, Allied submarines destroyed 1,200 merchant ships for about five million tons of shipping. Most were small cargo carriers, but 124 were tankers bringing desperately needed oil from the East Indies. Another 320 were passenger ships and troop transports. At critical stages of the Guadalcanal, Saipan, and Leyte campaigns, thousands of Japanese troops were killed or diverted before they arrived where they were needed. Over 200 warships were sunk, ranging from many auxiliaries and destroyers to one battleship and no fewer than eight carriers. Underwater warfare was especially dangerous; of the 16,000 Americans who went out on patrol, 3,500 (22%) never returned, the highest casualty rate of any American force in World War II. The Japanese losses, 130 submarines in all, were even higher. A single German submarine, U-862, operated in the Pacific Ocean during the war, patrolling off the Australian east coast and New Zealand in December 1944 and January 1945. It sank one ship in the Pacific before it was recalled to Batavia. Japanese Naval and Merchant Marine losses 1941-1945 can be found here at US Submarine claims of sunk/damaged ships 1941-1945 can be found at The Joint Army Navy Assessment Committee has assessed US Submarine credits.