The Submarine War in the Pacific:
American Success or Japanese Failure?

Thomas Shook

from War Studies Journal;
Spring 1996 • Volume 1 • Issue 2

It may be necessary to teach them how to use submarines.
Admiral Matome Ugaki,
16 January 1942

Assessing the primary reason for the United States' victory over Japan is quite simple. Japan did not have the material resources to fight a prolonged industrial war. Japan's material position - resting on the resources to which she availed herself from conquered territories - depended entirely upon the open sea routes of the southwest Pacific. Unlike the enigmatic results of Allied strategic bombing raids in the European theatre, the success of American submarine warfare substantially contributed to an Allied victory in the Pacific. As Japan's needs increased, her ability to acquire raw materials decreased. America's willingness to conduct unrestricted submarine warfare and Japan's inadequate response to that threat resulted in the loss of vital sea lanes and ultimately Japan's defeat.

US War Plans and Submarine Policy

The United States began planning for war with Japan as early as 1906. These early plans had their roots in the diplomatic controversies with the Japanese concerning immigration issues, but over several years they became War Plan Orange. This was an overall strategic plan for defeating Japan if she should begin an aggressive war for dominance in the Southwest Pacific.2 Early planners at the National War College believed that "... relentless blockade, destruction of ports and shipping, and...complete commercial isolation...would be the kind of war the United States could wage and win".3

Despite Plan Orange, the official American policy regarded submarines subject to the same cruiser warfare restrictions imposed upon surface ships. Captain Thomas Hart understood the effectiveness of submarines as a weapon of unlimited war, yet conditionally advocated their abolition as part of a larger arms agreement as early as 1919.4 Political support for limiting or eliminating submarines was a result of the general remonstrance against their unrestricted use by Germany during World War I. However, the Washington Naval Conferences ambiguous position on submarines left many in the United States dissatisfied.5 Most major powers continued to develop submarines but officially viewed them as a weapon designed to operate with the surface fleet, not as a strategic weapon for use against civilian commerce. It was within this policy framework that both American and Japanese Navies trained and drilled. Yet, despite this pre-war doctrine, the Navy Department transmitted the order to conduct "unrestricted air and submarine warfare against Japan" just six hours after the Pearl Harbor debacle on 7 December 1941.6

Two Major Problems

The US Navy had not trained submarine commanders to conduct this type of warfare. Four days after Pearl Harbor, a US submarine left there for the first war patrol. The Gudgeon, after approximately 21 days, arrived on station. After two disappointing contacts, the submarine located a small, unescorted coastal freighter. Using the standard attack procedure - submerged, making a sound approach 7 at no closer than one thousand yards - the submarine fired two torpedoes, both of which missed.8 This was a clear example of two major problems the US submarine force would have to overcome. The US Navy would have to adopt more aggressive tactics to secure hits and this in turn would lead to the recognition of problems with the torpedoes, which would have to be corrected.

Senior US naval officers were quick to recognize the problem of outmoded tactics. Captain Wilkes, Commander, Submarine Force Southwest Pacific, discarding cautious prewar tactics, radioed his boats to "Penetrate. Penetrate. Penetrate."8 Senior navy officers routinely sacked respected commanders, replacing them with younger, more aggressive skippers. The US Navy relieved thirty percent of its submarine commanders in 1942 for "non-productivity."9

As more aggressive commanders and better training took effect, the US discovered it had serious defects with its torpedoes. In June and July 1942, because of constant complaints from submarine skippers, tests conducted in the Pacific discovered that the mark XIV torpedo ran, on average, 11 feet deeper than set. It was not until August 1942 that the Bureau of Ordnance issued memoranda advising the fleet of the defect.10The fleet continued to have problems with both exploders. After numerous complaints about the unreliability of the Mark VI magnetic exploder, the fleet discontinued their use only to discover that the contact exploder was also defective. The US Navy did not fully correct these problems until September 1943.11 In the summer of 1943, the US introduced the Mark XVIII electric torpedo into the submarine fleet. It also had problems, particularly hydrogen build-up produced by the batteries, but they quickly became popular. Although slower than the Mark XIV, the Mark XVIII left no "trail of bubbles" to show from which direction it came. By the end of the war 65% of torpedoes expended were mark XVIII electrics.12

At the beginning of the conflict in 1941, American Submarines had radar - technology the Japanese would not have until later; it was, however, primitive. This first radar set, known as the type SD, was non-directional and had a ten mild range. It was designed to be used primarily to detect enemy aircraft, but many submarine commanders did not use it. They thought it was unreliable and would give away their position. Dave White, commander of the Plunger, though using the SD radar " Was tantamount to breaking radio silence." In the latter half of 1942, the US Navy began equipping its submarines with the new SJ radar. Although it required constant maintenance and calibration, it could provide the exact range and bearing of surface contacts at night and in all weather.14


Perhaps the most significant advantage the United States had was the ability to decipher Japanese codes. By September 1940 the US had broken the Japanese diplomatic code - "Purple" - and the primary Japanese navy code - JN25.15 This intelligence supplied the US submarine force commanders with accurate information on Japanese ship movements and allowed them to position submarines for maximum results.16 Between 7 and 9 May 1943, the US submarine Wahoo sank three Japanese freighters totally 9,660 tons using such intelligence.17 After the Japanese Navy began to organize convoys in 1943, the US Navy continued to gain valuable movement information from the broken Maru code used for convoy communications.18

Intercepted and decrypted intelligence allowed the United States to refine tactics by organizing "wolf packs" of submarines. Using information on Japanese ship movements to coordinate strikes from submarines moving in groups, the US inflicted some of the heaviest damage to Japan's shipping. In the first four months of 1944, American submarines sank 179 ships totally almost 800,000 tons. By the end of August this total would be more than one million tons, and by the end of the year imports of oil were at a virtual standstill.19 The remaining Japanese shipping was forced into coastal waters where American submarines had a difficult time operating.20 Allied forces had reduced the Japanese Merchant Marine to approximately 2.5 million tons, down from almost six million tons in 1941, "despite the addition of some 800,000 tons by conquest...and 3.3 million tons of new constructions".21

Perhaps the outstanding development of recent months has been the Coordinated Attack Group, sometimes known as the "Wolf Pack". Using to advantage the experience gained in the patrols in various groups, three submarine groups patrolled during July with considerable success. In these three groups, the average tonnage sunk and damaged by each submarine was about 27,000 tons. This compares favorably with an average of 9.336 tons per submarine in the war to date. These groups likewise appear to be the best answer to the increased antisubmarine protection given ships by the Japanese and to be the most practicable means of inflicting the maximum damage on the large, heavily escorted convoys now employed by the enemy.22

Chester Nimitz, CINCPAC
22 December 1944

Japanese Strategy

The Japanese high command had laid out a basic strategy for accomplishing their goals through military action. Rapidly seizing valuable areas in Southeast Asia would give them the resources to maintain a military capable of defending those areas against the United States, Great Britain, and the Netherlands. This rapid success of military operations would shift public opinion in the United States toward conciliation while "cooperation with Germany" would severe the "Anglo-American coalition."23 The Japanese Army Vice Chief of Staff, Ko Tsukada, stated:

If we take the South, we will be able to strike a strong blow against American resources of national defense. That is, we will build an iron wall, and within it we will destroy, one by one, the enemy states in Asia; and in addition, we will defeat America and Britain.24

These plans ultimately rested on propitious projections on the availability of merchant shipping. President of the Planning Board, Teiichi Suzuki, estimated that 3 million tons of commercial shipping would be sufficient to transport between 4.8 and 5 million tons of material per month. Furthermore, he stated,

...the maintenance of the three million tons of shipping...should be possible if we can obtain an average of about 6000,000 gross tons of new construction each year. [He further stated that this was possible] provided [the Japanese] lose [no more than] 800,000 to one million tons of shipping a year.25

The Japanese leadership, unlike their American counterparts, never planned an unrestricted campaign against American merchant shipping, nor did they plan to protect their own adequately. Japan, as an island nation with far fewer natural resources, was doubly dependent on her sea lanes, yet badly mismanaged available shipping and forces.

[Often] civilian ships went out in a ballast and returned loaded with raw materials. Army and navy ships went out loaded with troops and military supplies and returned empty...Not infrequently fleet destroyers streamed idly between parts, while important convoys moved independently over the same route unprotected.26

Japanese Submarine Technology and Operations

At the beginning of the War, Japanese submarine technology was at least equivalent to American. Japan had better torpedoes, exceptional optics and submarines capable of long-range operation.27 However, the Japanese viewed the submarine as an auxiliary to their fleet to be used against enemy warships. Merchant ships "...were legitimate targets only when there were no warships to be considered".28 This view severely restricted the effectiveness their torpedoes could have had against American matèriel.

When the war began, Japan had no less than three torpedo types in services which were vastly superior to any the United States would develop by the war's conclusion. The Type 89 could propel a 660-pound warhead at 45 knots for 6,000 yards. Several versions of the Type 92 electric were all able to deliver a 660-pound warhead traveling at 30 knots more than 7,500 yards. The Type 95 Mod 1 could send an 891-pound warhead 13,000 yards at 45 knots. Furthermore, Japanese torpedo exploders worked, unlike their American counterparts.29

The Japanese were making progress in submarine detection. By 1944 they had introduced planes equipped with radar and with magnetic airborne detectors (MAD) and air-dropped, circular running torpedoes. However, so few MAD equipped planes were available that they were called only after lookouts or radar had already sighted a submarine.30 Had the Japanese vigorously pursued more efficient anti-submarine warfare (ASW) methods very early in the War and placed more emphasis on the development and deployment of these new technologies, it might have taken a different turn.

Japan devoted much submarine resources to missions and designs which had little or no effect on the course of the war. The Japanese deployed both fleet and midget submarines at Pearl Harbor but with no success. Vice Admiral Shigeru Fukudome, an important planner of the Pearl harbor raid, "expected that more damage would be inflicted by submarine attacks...than by air attacks", when, in fact, submarines inflicted no damage.31

The Japanese used submarines to launch and recover long-range reconnaissance aircraft, often to the detriment of normal submarine operations. In May 1942, the crew of I-25 was preparing to launch a seaplane when the sopped an American cruiser but could not attack with the plane on the deck. That same month, the I-19 attempted to launch a seaplane when aircraft spotted it and forced the submarine to dive, abandoning the seaplane.32 In September 1942, the I-25 sent its seaplane on a futile mission to set the forests of Oregon on fire with incendiary bombs.33

Japan also diverted submarines to supply and transport duties. As early as July 1942, the Japanese experimented with various schemes to fire rice and other supplies from the torpedo tubes of submerged boats. To bolster the Japanese garrison on Guadalcanal, they recalled many of their fleet submarines and removed all but two torpedo tubes to accommodate more supplies, depriving the boats of precious offensive power. By January 1943 they had established a logistics train of about twenty submarines, "including the latest types", to support Japanese forces on the island. Ironically, these unorthodox efforts relieved pressure on the American fleet by helping to nullify Japan's advantage in torpedo technology, thereby aiding the US advance into Japan's defensive perimeter.34

However, even if the Japanese had conducted an unrestricted submarine campaign against US convoys, they could not have targeted the "internal" sources of raw materials available to the United States. In contrast, the Americans could (and did) strike at the vulnerable supply routes over which flowed virtually all of the raw materials required for the Japanese war effort. Japanese planners should have given a far greater priority to protecting their merchant sea routes.

The initial failure of the Japanese initially to protect their merchant shipping routes was not the fault of inadequate material resources or inferior weapons. They could effectively conduct antisubmarine warfare (ASW) actions as illustrated by an encounter with the US submarine Plunger had in January 1942. A Japanese destroyer, after initially seeing the submarine, prosecuted the contact using echo-ranging sonar and during the encounter accurately dropped some twenty-four depth charges. The US was not certain the Japanese had this type of sonar before this engagement.35 The fault lay with the attitude of senior Japanese Navy officers who felt,

Anti-submarine warfare was of little concern because the Japanese underestimated the submarine menace to shipping. At the beginning of the war, there were no naval units with the exclusive mission to escort merchant ships.30

There was little improvement during 1942. With no overall command structure to organize convoys, "Japanese merchant ships were frequently on their own."37

In the Spring of 1943, Japanese shipping lost to American submarines reached 100,000 tons per month.38 In November 1943 the Japanese formed the Grand Escort Command Headquarters which was responsible for all shipping except that in local waters. However, the new command had to compete with the Combined Fleet admirals for the allocation of units. "Hence, at the close of 1943, the best escort vessels in the Grand Escort Command were fifteen destroyers built in the early 1920's."39


Historians generally view the submarine war in the Pacific as a victory for the American submarine fleet, and it was. The US submarine force had several advantages which, when brought to bear against Japanese shipping, were decisive. America was ready and willing to carry out a campaign of unrestricted warfare from the very beginning. This included a willingness to replace conservative submarine captains with younger, more aggressive skippers with little regard to rank or standing. American commanders had radar, although primitive, from the beginning of the War. Despite faulty torpedoes, American skippers worked out effective tactics while energetically pursuing solutions to their technical problems. The greatest advantage the United States held was intelligence gained through broken Japanese codes. The ability of US officials not only to decipher the transmissions but also rapidly disseminate information to combat units was a crucial factor in the success of the submarine force.

The Japanese, however, were not simply victims of better technology. They contributed much to their own defeat. When the War began, they neither pursued American merchant shipping nor did they take adequate defensive measures to protect their own. As it became apparently how vulnerable their merchant traffic was, they failed to adjust to the realities of unrestricted warfare. Admiral Ugaki made clear his feelings that submarines were warships designed to engage other warships implicitly impugning the submarine's role in unrestricted warfare:

Based on a report of thirty enemy submarines heading north, the Second Destroyer Squadron and others carried out a submarine hunt one night, but there seems to have been no result. Have they turned back to the south? If so, they must be said to have no fighting spirit. It may be necessary to teach them how to use submarines.40

The Japanese also failed to develop the technology they did have to its full potential.

Diverting submarines way from hunting and sinking ships to various other unproductive tasks not only wasted material resources but denied their submarine captains valuable time to develop better tactics and offensive warfare skills. It also denied the Japanese the opportunity to exploit their advantage in torpedoes.

The Japanese were in a difficult material situation which their own oversights often made worse. They understood the necessity of a continued flow of resources over vital sea-routes. Navy Chief of Staff Nagano urged a quick decision about whether or not Japan would go to war. He declared,

The Navy is consuming 400 tons of oil an hour. The situation is urgent. We want it decided one way or the other quickly.41

Despite this understanding, the Japanese never took adequate measures to protect these resources once acquired. In the end, the efforts of the American submarine force helped to decide the outcome but not without some assistance provided by the oversights and missed opportunities of the admirals of the Japanese Combined Fleet.

Thomas A. Shook, University of South Carolina, served six years in the US Navy and another six years as a civilian analyst with the Polaris Material Office.

1 Charles A Lockwood, Sink 'em All: Submarine Warfare in the Pacific (New York: E P Dutton, 1951), p. 351.

2 Edward S Miller, War Plan Orange: The US Strategy to Defeat Japan, 1897-1945(Annapolis: The united States Naval Institute, 1991), p. 22.

3 ibid., p. 28.

4 Janet M Manson, Diplomatic Ramifications of Unrestricted Submarine Warfare, 1939-1941 (New York: Greenwood Press, 1990), p. 38.

5 ibid., pp. 41-42.

6 Clay Blair, Silent Victory (Philadelphia: J B Lippincott, 1975), p. 106.

7 W J Holmes, Undersea Victory: The Influence of Submarine Operations on the War in the Pacific (New York: Doubleday, 1966), p. 13.

8 ibid., p. 87.

9 Blair, p. 553.

10 ibid., pp. 277-278.

11 ibid., p. 20.

12 Lockwood, pp. 105-107.

13 Blair, p. 113.

14 ibid., pp. 321-322.

15 ibid., pp. 73-74.

16 ibid., p. 18.

17 ibid., p. 426.

18 Ronald Lewin, The American Magic: Codes, Ciphers and the Defeat of Japan, (New York: Farrar, Straus & Giroux, 1982), p. 223.

19 ibid., pp. 228-229.

20 Blair, p. 819.

21 Samuel Eliot Morison, The Two Ocean War (Boston: Little, Brown, 1963), p. 511.

22 Nobutaka Ike (ed.), Japan's Decision for War: Records of the 1941 Policy Conferences (Stanford: Stanford University Press, 1967), p. 153. Prepared reference materials for the Imperial Conference 6 September 1941.

23 ibid., p. 207. Imperial Conference of 5 November 1941.

24 ibid., p. 215.

25 Holmes, pp. 97-98.

26 ibid., pp. 194-195.

27 Mochitsura Hashimoto, Sunk: The Story of the Japanese Submarine Fleet, 1942-1945, translated by E H M Colgrave (London: Cassell, 1954), p. 33.

28 Dorr Carpenter & Norman Polmar, Submarines of the Imperial Japanese Navy,(Annapolis: The Naval Institute Press, 1986), pp. 156-157.

29 Holmes, pp. 357-358.

30 Carl Boyd, "The Japanese Submarine Force and the Legacy of Strategic and Operational Doctrine Developed Between the World Wars', in Larry H Addington et al. (eds.), Selected Papers from the Citadel Conference on War and Diplomacy, 1978, (Charleston: The Citadel Development Foundation, 1978), p. 28.

31 Holmes, p. 130.

32 ibid., p. 169.

33 Hashimoto, pp. 60-63.

34 Blair, p. 113.

35 Boyd, p. 33.

36 Holmes, p. 195.

37 Blair, p. 424.

38 Boyd, p. 33.

39 Ugaki, p. 76. Diary entry of 16 January 1942.

40 Ike, p. 186, The 59th Liaison Conference of the Imperial Cabinet, 23 October 1941.

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