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Submarine warfare

US submarines, as well as some British and Dutch vessels, operating from bases at Cavite in the Philippines (1941–42); Fremantle and Brisbane, Australia; Pearl Harbor; Trincomalee, Ceylon; Midway; and later Guam, played a major role in defeating Japan, 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, Japanese oil supplies were so limited that its fleet was virtually stranded.

The Japanese military claimed its defenses sank 468 Allied submarines during the war. In reality, only 42 American submarines were sunk in the Pacific due to hostile action, with 10 others lost in accidents or as the result of friendly fire. The Dutch lost five submarines due to Japanese attack or minefields, and the British lost three.

American submarines accounted for 56% of the Japanese merchantmen sunk; mines or aircraft destroyed most of the rest. American submariners also claimed 28% of Japanese warships destroyed. They also played important reconnaissance roles and rescued hundreds of downed fliers.

Within hours of Pearl Harbor, in retribution against Japan, Roosevelt promulgated a new doctrine: unrestricted submarine warfare against Japan. This meant sinking any warship, commercial vessel, or passenger ship in Axis-controlled waters, without warning and without aiding survivors. The US had a long supply line, leaving it vulnerable to submarine attack, but Japan used its submarines primarily for long-range reconnaissance and only occasionally attacked US supply lines.

As the war turned against Japan, IJN submarines increasingly served 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 American freighters shipping military supplies from San Francisco to Vladivostok, much to the consternation of its German ally.

The US Navy, by contrast, relied on commerce raiding from the outset. However, the problem of Allied forces surrounded in the Philippines in early 1942 led to diversion of boats to "guerrilla submarine" missions. Basing in Australia placed boats under Japanese aerial threat while en route to patrol areas, reducing their effectiveness, and Nimitz relied on submarines for close surveillance of enemy bases. The standard-issue Mark 14 torpedo and its Mark VI exploder both proved defective and were not corrected until September 1943. Before the war, a US Customs officer had seized a copy of the Japanese merchant marine code, not knowing that the Office of Naval Intelligence had broken it. The Japanese promptly changed it, and the new code was not broken again by OP-20-G until 1943.

In 1944 the US Navy restructured their submarine fleet, installing effective shipboard radar on its 150 submarines, replacing commanders deemed lacking in aggression, and fixing the faults in the torpedoes. Japanese commerce protection was "shiftless beyond description", and convoys were poorly organized and defended compared to Allied ones. The number of American 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 of Japanese vessels had decreased because there were less ships available to target. In all, Allied submarines destroyed 1,200 merchant ships – about five million tons of shipping. At critical stages of the Guadalcanal, Saipan, and Leyte campaigns, thousands of Japanese troops were killed or diverted from where they were needed. Over 200 warships were sunk, including a 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 higher.