JAPANESE SUBMARINE STRATEGY AND TACTICS, 1937-41

“I-25 Shells Fort Stevens”. On the night of June 21, 1942, the Japanese submarine I-25 fired 17 rounds at Fort Stevens, Oregon located at the mouth of the Columbia River. It was the second attack on a military base in the continental U.S. during WWII [Painted by Richard L. Stark]

Japanese type B-1 submarine cutaway, ca-1944

The 1920s and 1930s had seen much submarine construction notwithstanding efforts to outlaw or curtail submarine operations by international law. Britain, in spite of its determination to abolish the submarine as more a threat than an asset to its maritime security, had continued to build submarines and to experiment in their design. The United States maintained a growing number of submersibles, though until the mid-1930s these were mostly smaller boats for coastal defense. Lesser naval powers saw the submarine as a means of compensating for the modest size of their battle fleets. France remained the foremost advocate of the submarine, seeing it as a fleet equalizer, and had embarked upon a considerable submarine construction program, which included the construction of the huge Surcouf. Germany had begun secret preparations to rebuild its submarine fleet in the 1920s, an effort so successful that in 1935, less than a week after signing an accord with Britain that generally freed it from restrictions on construction in this category, German launched its first submersible. Japan had led the way in the construction of oceangoing submarines, predicating its submarine policy on the strategic principles set forth by Admiral Suetsugu.

If the pace of submarine construction quickened in the 1930s, at the end of the treaty era, naval strategists worldwide held no consistent views about the most advantageous use for submarine fleets. Certainly, none of the submarine strategies of the former maritime Allies of World War I was shaped by the evidence of the awesome destructiveness of the submarine as a commerce raider. Officially, the United States, up to the outbreak of the Pacific War, continued to view the submarine primarily as an element of fleet operations, though influential submariners in the U.S. Navy appear to have privately advocated a policy of aggressive commerce raiding in the event of war with Japan. Though Britain was determined to stay abreast of the latest developments in submarine design and technology, it failed to develop a coherent submarine strategy, partly because of financial stringency and partly because of the Royal Navy’s continuing priority on the battle fleet. In the Pacific, this left Britain with a submarine force inadequate in numbers and range to carry offensive war to Japan. The submarine force was thus relegated to the defense of the base at Singapore, though World War I offered little evidence that submarines were effective in a defensive role. Given the political and military realities of the time, particularly the near-at-hand Italian threat in the western Mediterranean, French thinking behind the construction of Surcouf seemed, by the late 1930s, quite misconceived. Among the Western maritime powers, only the small but resurgent German navy began to assemble a submarine force whose principal objective was the destruction of enemy commerce. Against the convoy system, which had finally turned back the German U-boat offensive in World War I, Comdr. Karl Donitz now devised new wolf-pack tactics that called for the massing of submarines at night and on the surface.

In part, the coordination of such tactics was made possible by greatly improved communications between submarines and shore command. High-frequency radio transmitters ashore now made it possible to send messages to submarines at great distance from the land, and extremely powerful, very low frequency (10-20 kilohertz) signals could be received even by submerged submarines. Though the vessels had to surface to transmit, these new developments in radio communication not only enhanced the value of the submarine for reconnaissance, but allowed more effective control of submarine fleets. Of course, radio direction finding, perfected during the late 1930s, enabled an enemy to detect the location of a submarine transmitting messages, which was a key element in the development of antisubmarine warfare (ASW) in World War II.

In the mid-1920s, Adm. Suetsugu Nobumasa had given the Japanese submarine force several missions that transformed it, in theory, into a long-range offensive system. The missions assigned to this system were the extended surveillance of the enemy battle fleet in harbor, the pursuit and shadowing of that fleet when it sortied from its base, and the ambushing of the enemy by pursuing submarines that would destroy a number of his capital ships and thus reduce his battle line just before the decisive surface encounter with the Japanese battle fleet.

By 1930, this strategy, embodying the principles of extended, long-range surveillance, pursuit, ambush, and attrition of the enemy, had become, like the 70 percent ratio in the 1920s, an article of faith in Navy General Staff planning for war with the U.S. Navy. Yet, surprisingly, for reasons not entirely clear, the strategy had never been subjected to the trial of its various tactical elements. This failure stands in marked contrast to the navy’s rigorous testing of other tactical matters. At all events, late in the decade, with the acquisition of large submarines with high surface speed, starting with the J3 type, the navy finally began frequent and intensive training in the tactics of surveillance, pursuit, shadowing, and ambushing of enemy fleet units.

Such training began in 1938 with a series of exercises designed to test the efficiency of submarines and crews for intense periods of patrol near the enemy. The next year, the navy began serious practice in submarine attack doctrine, beginning with close surveillance of tightly guarded heavy surface units, both those in harbor and those under way. The results were disconcerting, to say the least. Trying to get near to fleet targets, some submarines taking part in the exercises strayed within waters patrolled by destroyers and were judged to have been sunk; others apparently gave away their positions through radio transmissions. Still others, which had remained undetected while submerged during the periods of most intense antisubmarine activity by destroyers and aircraft, nevertheless missed important radioed instructions.

From these exercises the navy drew several conclusions that eventually were translated into standard operating procedures during the Pacific War. Unhappily for the Japanese submarine force, none made for effective submarine strategy, and some were downright disastrous. The great stress on concealment during extended surveillance operations in enemy waters seems commonsensical, but during the war it contributed to the extreme caution of Japanese submarine commanders off American shores. It also explains the Japanese wartime practice of using submarine- borne aircraft, particularly during moonlight nights, to reconnoiter enemy harbors and bases in place of submarines themselves. (While this technique was used during the war for reconnaissance over several Allied bases, it failed to produce any significant operational results.) Undoubtedly, however, the most significant lesson to come out of the 1938 exercises was the extreme difficulty in maintaining close submarine surveillance of a distant and carefully guarded enemy base. This was the first of many revelations pointing to the clear unworkability of accepted Japanese submarine doctrine.

The key to the success of the actual interception operations was the proper deployment of submarines for the best possible torpedo shots against an advancing enemy fleet. Experience had shown that the best position for a torpedo shot was a 50- to 60-degree angle off the bow of the target at a distance of about 1,500 meters (1,650 yards). Even if the submarine commander was slightly off in his estimates of target course and speed, and even if the target changed course, the likelihood of a hit was greatest with this position. For a submarine to arrive at this optimum point, it needed maximum freedom of movement, so as to locate itself across the enemy’s intended course. Where the enemy’s course was known, interception operations called for pursuing submarines to race ahead of the enemy fleet to a spot where they could lie in wait and maneuver themselves into an ideal firing position. Where the enemy’s actual course was unknown, a picket or ambush line was to be thrown across the track that the enemy seemed most likely to follow.

In 1939 and 1940, as part of a series of maneuvers held in the western Pacific from Honshū to Micronesia, Japanese submarines began to practice these tactical requirements for long-distance interception operations. In these exercises an “A Force” was usually designated to defend Micronesia from an invading “B Force” coming down from Japan. Once B Force had sortied from its home base, A Force was supposed to “acquire” it, pursue it, maintain contact with it, and then destroy it in ambush. To their dismay, the A Force submarine commanders discovered that despite their surface speed, they could just barely maintain contact with the advancing B Force. It proved difficult to race ahead of the enemy and then lie in wait in an ideal firing position, particularly since they were obliged to fire while submerged and, once underwater, they were practically stationary. The surface targets all too often raced by unscathed. Firing on the surface seemed out of the question, since the submarines were easily spotted not only by patrolling destroyers but by carrier-borne aircraft used in an ASW role.

These maneuvers also supplied the navy with one irrelevant “lesson” and another a good deal more ominous. Because of the activity of submarines as well as land-based bombers and flying boats in the defense of Japanese-held atolls in Micronesia, the navy became convinced of the value of both submarines and aircraft in the defense of Japanese island bases in the western Pacific. In fact, submarines were little use in island defense in the Pacific War, except as supply ships for stranded garrisons, and of Japanese aircraft there were never enough, once the American amphibious wave crashed into Micronesia late in 1943. More to the point, the maneuvers demonstrated to individual submarine captains the near impossibility of the pursuit-contact-annihilation elements of the interception strategy, as well as the hazards of these tactics, so long the mainstays of Japanese submarine strategy.

Because the Japanese navy never did assemble its projected long-range attack groups, it is impossible to know what sort of tactics and command structure might have developed out such a combat organization. What is clear from the maneuvers of 1939-40 is that although the navy held exercises bringing together groups of submarines, it never developed the idea of concerted attack. Specifically, the German (and American) “wolf pack” concept—whereby an embarked commander directed multiple attacks on common targets by submarines under his command—apparently never occurred to those directing the Japanese submarine forces. The Japanese approach to submarine operations, practiced in prewar maneuvers and carried out during the Pacific War, was to retain control of submarine forces from a shore command. The navy’s failure to produce more than three type A1 command submarines meant that the at-sea command concept built around Ōyodo-class cruisers never materialized. An ambush or picket line of submarines might be established across an anticipated course of enemy advance, for example, and submarines assigned stations along it, but once on station, a submarine was generally moved only by orders from ashore. This failure of the Japanese submarine forces to develop either the concept of concerted attack or the skills and command structure to make it work is another reason for the meager results of Japanese submarine operations during the war.

The doctrine of concerted attack was developed by the German and U.S. navies for commerce raiding, not for attacks on major fleet units. Moreover, from Suetsugu’s day forward, Japanese submarine doctrine clearly focused on the enemy’s battle fleet, not primarily his sea communications and commerce. The Japanese navy, however, was not entirely unaware of the possibilities of commerce raiding. In maneuvers in October 1940, the Japanese navy actually deployed several submarines to patrol vital maritime corridors in the home islands—Tsushima Strait, between Honshū and Korea; Bungo Strait, between Shikoku and Kyūshū; and Uraga Strait, the entrance to Tokyo Bay. The submarines simulated attack on Japanese commercial vessels to determine how vulnerable the commercial fleet was to submarine raids. Because of inadequate Japanese antisubmarine capabilities and lack of attention to convoy escort, in only five days, 133 Japanese merchantmen were “sunk” by the submarines involved in these simulation exercises. Considering that four years later, these same waters were the scene of actual havoc and slaughter by American submarines, one can only wonder why the lessons of this exercise were not more salutary. Unfortunately for the Japanese, the principal conclusion drawn by those in command related not to the offensive potential of attacking submarines, but to their vulnerability to detection by radio direction finding.

Not surprisingly, the Japanese navy’s neglect of the extreme vulnerability of the Japanese home islands to submarine blockade was matched by its general disinclination to give priority to a submarine campaign against American coastal and trans-Pacific shipping. While the high command did acknowledge that threatening the enemy’s sea lanes was an important part of naval warfare and that submarines should take part in such operations, it held that they should do so only as long as such operations did not greatly interfere with their primary mission of destroying enemy fleet units in battle.

As discussed, a certain incoherence in Japanese submarine strategy was shown by the multiple submarine designs developed before the Pacific War. A further fragmentation of that strategy occurred in 1940-41, now reinforced by the difference in submarine types. It began with the reorganization of the submarine forces late in 1940. The navy created a separate submarine fleet, the Sixth, composed of the first three of the navy’s seven submarine flotillas. The other flotillas were distributed to the Combined, Third, and Fourth Fleets. Each submarine flotilla began operational training according to the mission or missions it had been assigned. As each flotilla was usually composed entirely of submarines of one type, that type differing from flotilla to flotilla, missions had to be shaped by the capabilities and limitations of the particular type. This tactical reality was demonstrated in exercises held in May 1941 that were designed to test different types of submarines in different operational situations. The cruiser submarines proved themselves sluggish but reliable and capable of great endurance. They were therefore confirmed as suitable for long-distance operations—attacks on enemy bases, disruption of enemy transport routes, and ambush operations—but were judged not likely to be effective in fast-moving fleet attacks. On the other hand, the Kaidai submarines, with their slightly greater speed, would be used to pursue, shadow, and attack a westward-moving American fleet or could be deployed in the vanguard of a counterattacking Japanese surface force once the enemy reached Japanese waters.

Exercises undertaken by the Sixth Fleet’s Second Submarine Flotilla from February to April 1941 in the waters between Honshū and Micronesia revealed further difficulties in the pursuit, contact-keeping, and attack formula so long the accepted submarine doctrine in the navy. The postexercise report of the flotilla’s staff pointed out some major problems in the approved strategy. Chief among them were insufficient numbers in surveillance operations, the risk of discovery by antisubmarine forces, and inadequate submarine speed. The current strength of advanced submarine forces was simply insufficient to monitor effectively an enemy fleet in a distant harbor. Moreover, the report argued, submarines assigned this mission had to be positioned at sufficient distance to avoid antisubmarine patrols. Consequently, the enemy fleet often sortied from base undetected by the patrolling submarines. Once at sea, submarines in pursuit continued to have difficulty in maintaining contact with the enemy and even greater difficulty in getting into firing position to attack him. As in previous years, problems were encountered in setting up an ambush or picket line; once again, with submarines stationed along the line at extended intervals, the enemy too often slipped by. To deal with these various problems, the staff of the Second Submarine Flotilla recommended that in gathering information on fleet departures from enemy harbors, the navy should depend as much upon Japanese intelligence organizations as it did on submarine surveillance. To monitor the progress of a westward-moving American fleet, the navy should deploy lines of fishing boats across the enemy’s anticipated track, backed up wherever possible by large flying boats, based, most likely, in Micronesia. These recommendations are a dismal commentary on the general failure of the interception concept by 1941. Certainly, the staff of the Second Submarine Flotilla no longer believed that submarines by themselves could significantly reduce an American battle fleet not yet within the range of Japanese naval and air bases.

Further testing of equipment and training of crews in 1941 extended the scope of problems relating to the use of submarines in the decisive fleet encounter. In March and July, submarines of the Sixth Fleet and Fourth and Fifth Submarine Flotillas practiced close-in attacks on well-guarded fleet units. Again, however, such operations all too often caused the attacking submarines to be discovered because their periscopes were sighted. Some submarine officers suggested that because of the difficulties in carrying out such attacks, it would be better to carry out long-distance firing than to have the attacking submarines discovered and have the enemy turn away to avoid Japanese torpedoes. Though long-distance firing had been conceived in the 1920s, remarkably, its effectiveness had never really been tested. In any event, the concept of long-distance firing cut across the grain of the navy’s traditional nikuhaku-hitchii spirit of close-in attack. This problem lay unresolved by the time the war broke out in December 1941.

Accounts of the Japanese submarine exercises of 1939-41 clearly show that Japanese navy training attempted to be comprehensive, rigorous, and innovative in the development of the submarine as a weapon to attack regular fleet units. Tactical training was carried out at night as well as during daylight. Long- and short-distance operations were practiced. Coordination was attempted between submarines and aircraft. New weapons, like the type 95 torpedo, were tested, and novel techniques, such as “submerged firing,” were practiced.

Japanese submarine commanders developed zembotsu hassha (submerged firing) because, although their boats had proper range finders and devices for ascertaining exact bearing, they could only be used on the surface. Preferring to stay submerged when attacking fleet units screened by destroyers, submarine commanders would submerge, expose the periscope for a final optical reading of bearing and range, keep the periscope down for the final closure to the firing point, and then fire on sound bearings. This technique was hardly unique to the Japanese navy; American submarine commanders practiced something very much like it (“sound shots”) before the war, but having little luck, dropped them.

Despite the navy’s intensive training and development of new tactics, by the eve of the Pacific War, the navy had apparently not resolved the central problem of submarine tactics: the opposing requirements of self-preservation and aggressiveness. The essence of the submarine—and its preservation—is stealth. To be combat-effective, however, it must reveal itself at the moment of attack. “The trade-off between preservation and combat effectiveness,” Norman Friedman has written, “is central to submarine tactics and submarine design.” Right up to the Pacific War, Japanese submarine forces held to the navy’s traditional aggressive “close-in-sure-shot” torpedo tactics rather than switch to long-distance firing (and thus was a notable exception to the navywide obsession with outranging the enemy). At the same time, however, Japanese submarine commanders favored the contradictory and passive emphasis on concealment, which meant staying submerged for as long as possible, lying in wait for heavy enemy fleet units to come steaming by and present themselves as targets. By the time the war began, the concern for concealment proved stronger than the necessity for aggressive action, which, in the prewar exercises, appeared to lead all too often to discovery by antisubmarine surface and air forces. The result was a submarine force hobbled by conservative doctrine and aimed primarily at the destruction of naval targets. After the war, American naval interrogators were astounded at evidence of the timidity of Japanese submarine commanders on patrol. “It was frankly impossible to believe that [Japanese] submarines could spend weeks on the U.S. west coast ‘without contacts,”’ one analysis recorded caustically, “or spend more than forty days among the Solomons during the Guadalcanal campaign ‘without seeing any targets.’”

In fairness, it should be pointed out that before the war, U.S. submarine tactics showed a similar passivity. Submariners came to almost identical conclusions regarding the vulnerability of submarines attacking battle formations based on fleet exercises before the war. They believed, for example, that exposing the periscope in a fleet action was suicide, and they practiced firing on tracks based on bearings acquired by sound detection. The poor performance of U.S. submarines at the beginning of the war is partly attributable to these ingrained tactical lessons, and only the shock of American losses in the Pacific led to rapid adjustments in submarine doctrine.

For the Japanese submarine forces, in any event, this excess caution undoubtedly stemmed from lessons of the exercises of 1939-41, which revealed the extreme difficulties of carrying out their longstanding missions. These difficulties arose in part from the design of the vessels themselves, but even more from the impracticability of the prescribed tactics for the various missions. In the view of one former submarine commander, the fundamental reason for the failure of the Japanese submarine forces in the Pacific War was that those who made the basic decisions on submarine tactics—staff officers in the Combined Fleet and on the Navy General Staff—were ignorant of both the capabilities and the limitations of submarines. Typifying this lack of practical submarine knowledge was Admiral Suetsugu, who can be thought of as the father of Japanese submarine strategy. Because Suetsugu was a gunnery specialist, not a submariner, the essential strategies and tactics that he had devised for Japanese submarines were, in practice, unworkable. Individual Japanese submarine commanders knew that the tactics were impractical by the opening of the war, but being loyal and courageous officers, they did their job as best they could. During the war, this gap between staff ideas and combat realities grew even greater, and by war’s end, despite enormous losses, the Japanese submarine force had substantially failed to affect the course of the war.

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