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TM-E 30-480: Handbook on Japanese Military Forces
Technical Manual, U.S. War Department, October 1, 1944
[DISCLAIMER: The following text and illustrations are taken from a WWII U.S. War Department Technical Manual. As with all wartime manuals, the text may be incomplete or inaccurate. No attempt has been made to update or correct the contents of the original technical manual. Any views or opinions expressed do not necessarily represent those of the website.]

Chapter VII: Tactics of the Japanese Army

Part II: Application of Tactics

Section X: Japanese Coastal Defense

1. GENERAL. a. Scarcity of information. (1) Coast defense, as referred to in this section, is generally the method of defense adopted for large island and mainland areas, where defended localities are placed in strategic positions.

(2) The information on Japanese permanent or fixed coastal defense methods is very meager. However, thorough study of their training regulations, aerial photographs, and defensive measures carried out in the North and Southwest Pacific Areas as well as in Burma, present a fairly clear pattern of their methods for semi-permanent installations. As yet it is impossible to give a complete analysis of this subject, since it is only comparatively recently that it has been possible to draw on active combat experience. Furthermore, it is to be expected that Japanese technique is not as yet fully developed, and changes will no doubt ensue. It is believed, however, that the general methods and defensive layouts described and illustrated herein will be of value in estimating the form coastal defense may take in the future.

b. Doctrine. (1) It must be borne in mind that the Japanese conception of defense is essentially offensive, consequently they do not envisage coast defense as a passive process of merely resisting a hostile attack, but rather one offensive in nature, whereby the enemy forces must be attacked and destroyed before landing or as close thereafter as possible.

(2) A natural corollary of this offensive attitude is the determination not to surrender, but to fight on to the last man and the last round. An excellent example both of this characteristic and of the offensive spirit was given by the Japanese defenders of Attu in the Aleutians. Here, after stubborn fighting, the defenders were ejected from their prepared positions, and those that remained were driven back and thrown into an obviously hopeless situation. However, rather than surrender, all of the able and wounded alike joined in a final desperate charge which was only stopped as the last of them was killed. This tenacity of the Japanese in defense must always be reckoned with when calculating the resistance to be expected from a coast defense position.

2. PERMANENT FORTIFICATIONS. a. General. The fixed fortifications of Japan proper are primarily coast defenses. Although very little information as to age or character of gun complement has been obtained, there are indications that some modern guns as large as 16-inch caliber are emplaced in some areas. Antiaircraft guns are included in the plan of defense.

[Figure 115. A coast defense position.]
Figure 115. A coast defense position.

b. Mission. The coast defense system is the second line of defense, designed to insure the security of the Empire, independently of the fleet. Defense zones are located throughout the main islands to guard strategic areas. Army and Navy aircraft act in concert in coast defense, although the primary responsibility is recognized to be that of the Navy.

c. Mines. Great reliance is placed on mining operations which are a function of the Navy. The Japanese envisage the extensive use of mines to cover the entrances of all approaches along their coast and to block entrances to the narrow seas between Japan and the Asiatic mainland. This inner edge of the mined area is to be patrolled by surface craft and aircraft from nearby fields to prevent mine sweeping operations. Inner mine barrages will be laid covering strategic points, and these will be covered by shore batteries.

3. ATTU ISLAND DEFENSE PLAN. a. Figure 116 shows that portion of Attu Island over which fighting took place during its recapture by United States forces in the spring of 1943, and it also gives some idea of the mountainous nature of the terrain.

[Figure 116.  Map of Attu Island.]
Figure 116. Map of Attu Island.

The Japanese defense plan for Attu Island seems to have been based entirely on the assumption that any Allied attack would take the form of a landing in the main bays and subsequent advance up the valley beds. The guns and positions covering the main northern bays were so sited and concentrated that landing craft would find it extremely difficult to reach shore while one gun still remained to fire. All positions were well emplaced, and the guns were secure against everything except a direct hit. In addition, in order to meet the contingencies that hostile naval fire and air bombardment might knock out the dual purpose guns, and a beach landing might be made, the Japanese had prepared almost innumerable positions flanking the Holtz Bay beaches and facing inward and even to the rear. Behind the most inviting landing beaches they apparently had prepared 4 successive lines of resistance, with a final defensive line at the valley head. The setting for their plan was completed by the defense layout in Massacre Valley, where the lower positions flanking the valley were hidden by excellent camouflage, while those higher up were shrouded by the prevailing mist and clouds.

b. From the evidence of the siting of these positions, it is reasonable to assume that the Japanese appreciation of the course of the battle for Attu was that either a direct frontal assault would be made on the Holtz Bay beaches, or a back-door approach would be attempted via Sarana or Massacre Bays, the latter seemed more probable, for the exit from Sarana is blocked by Lake Nicholas. Their plan for Holtz Bay was designed chiefly to annihilate the attacking forces on the water or on the beaches and then to exterminate the few that might penetrate into the valleys by concentrated enfilade fire. On the other hand, the apparent intention for Massacre Bay was to meet attack by more subtle means. Because of good camouflage and cloud cover, few positions in that area were likely to be located by previous reconnaissance, and the few visible tents, trails, and fox holes would suggest that the valley was only thinly out-posted. The few scattered soldiers who fled the beach at the approach of the attacking force may well have been a decoy to invite an advance up the bed of Massacre Valley. Against that march the enemy probably only intended to use enough frontal resistance to ensure the building up of a powerful force. When that force was pocketed in the valley, all the flanking machine guns and mortars would open fire, grenade discharger and rifle fire would plunge down from above, and finally the artillery at the head of the valley would complete the process of annihilation. The Allied Forces thus would be pinned to the ground and destroyed in detail by inferior forces having the two decisive advantages of concealment and absolute command of terrain.

c. The plan of defense for Attu failed, as do so many Japanese plans, because it made no provision for the unexpected. The direction of the attack of the Northern Force (shown on the map) took them by surprise and out-flanked their carefully prepared Holtz Bay positions which lacked adequate all-round defense. The direction of attack of the Southern Force from Massacre Bay had been anticipated correctly, but not the nature of its execution which involved the quick movement to high ground outflanking and dominating Japanese positions. The defense plan met some slight initial success, but was frustrated completely when Allied troops took the initiative by fire and movement tactics and promptly seized dominating terrain features.

4. KISKA DEFENSES. a. General. (1) The garrison at Kiska was composed of Army and Navy personnel in about equal numbers. Although a few Naval personnel were found at Attu, the garrison was composed almost entirely of Army men, and consequently the barracks, weapons, etc., were all of Army type. At Kiska the Navy was concentrated around Kiska Harbor, while the Army occupied the area around Gertrude Cove. Barracks and weapons of the Navy differed in some respect from those of the Army. In the future, particularly in the North Pacific Area, it is reasonable to expect installations similar to those on Kiska, where both the Army and Navy occupied different sections of the same island.

(2) The Japanese development of Kiska was much more extensive than had been the development of Attu. Almost all beaches possessed some defenses including barbed wire and mines. In addition to the gun types found at Attu (75-mm and 20-mm AA, 75-mm and 37-mm Mtn. artillery, and small arms), Kiska ordnance included 6-inch. 4.7-inch, and 76-mm naval CD guns, 25-mm and 13-mm (single and twin mount) antiaircraft guns, and 3 light tanks. Heavy machine guns in a few cases were mounted in concrete pill boxes. Passive defenses included a radar installation, two 150-cm searchlights, and two 98-cm searchlights. Medical facilities were housed in well equipped underground hospitals.

(3) In contrast to undeveloped Attu, Kiska defensive areas were linked by a fairly well developed road net. Nearly 60 trucks, 8 sedans, 20 motorcycles, and 6 bantam-sized autos operated over this system. Two small bulldozers, tractors, and rollers were available for work on the airfield. The submarine base, the seaplane base, 2 machine shops, a foundry, and a saw mill complete the list of Kiska's special installations.

(4) Water and power systems were well established at Kiska in contrast to rather primitive systems employed at Attu. Power was provided mainly by 3 large power houses, but many additional smaller units supplied special buildings and outlying areas. Water from half a dozen small reservoirs was piped to installations and fire hydrants throughout the Main Camp area. Three complete radio stations, a radio-type navigation aid, and a well installed telephone system made up the communication network.

b. Beach defenses. (1) For the most part the Kiska shoreline is lined by steep cliffs, with sand or gravel strips at the heads of the many coves which indent its bluff line. Exits from these gravel beaches are provided by steep stream valleys that rise abruptly between towering hills to the high central ridges of the interior. In emplacing his beach defenses the Japanese made excellent use of this naturally rugged terrain. Nearly all of the usable beaches at Kiska possessed at least light defenses, and in the island's strategic mid-section beaches that gave access to the built-up areas around Kiska Harbor and Gertrude Cove were strongly defended.

(2) In general, Kiska beaches, which were accessible to landing craft, were mined; tank traps blocked possible overland exits; and barbed wire barriers were strung between breaks in the bluff line. From high ground at each of their extremities most beaches were completely covered by well camouflaged machine gun dugouts and rifle pits. In the hills behind these beaches other covered machine gun positions and trench systems with numerous fire bays commanded all possible exits along valley routes. At a few of the strategic coves single 75-mm and 37-mm artillery pieces were established in covered emplacements that commanded both the beach landing and its water approaches. In the more thickly settled areas like the Main Camp and the Sub Base the shoreline was honeycombed with dug-in machine gun positions, and in a few cases defenses were bolstered by reinforced concrete pill boxes.

5. THE DEFENSE OF A PROMONTORY. a. General. (1) The Japanese positions, shown in Fig. 117, have been located by aerial photography alone, therefore it cannot be assumed that the picture is complete in all respects. Sufficient information seems to be available, however, to make clear the general pattern of defense, namely, the concentration of positions on the high ground so as to provide for coast defense, by means of fire power directed from well constructed positions on dominant ground, and to afford direct defense for the two landing strips. The exception to this principle is the siting of defense positions (Position B) close to the water line in the bay on the North-east of the promontory; this, however, can be explained by the relatively low-lying ground inland in this area, which necessitates the more forward placing of positions. It can be expected that many, if not all, the antiaircraft guns shown have been sited with a view to use in a dual-purpose capacity.

[Figure 117. The defense of a promontory.]
Figure 117. The defense of a promontory.

(2) There seems to be a similarity in the general defense layout between this system and that of Attu Island, except in the important point of camouflage. That little effort at concealment has been made in the present instance is clear from the amount that aerial photography has been able to reveal, whereas on Attu Island close attention was paid to camouflage and a good degree of concealment was achieved.

b. Development Area A. (1) Figure 118 illustrates the various stages in the construction of a defense position sited on a spur of high ground giving good fields of fire over the neighboring beaches. The locality is that shown at position (A) overlooking the main bay from the East at the north end of the promontory shown in figure 117.

[Figure 118. Stages in the construction of a defense position.]
Figure 118. Stages in the construction of a defense position.

(2) The earliest constructional activity consisted only of ground clearance, the marking out of the trench system, and a little preliminary digging. A month later the position was already fairly well advanced, for by then the trench system, up to a length of some 700 yards, had been completed, and 3 strong points had been incorporated in the general layout. The developments in the position which were seen to have taken place several months later are indicated as Stage 3. As can be seen, its defenses have been improved by additional trench digging, while a fourth strong point has been added, and wire defenses also have been erected. In addition, the communications have been improved by the making of a new trail into the position. In its final form, the locality can be deemed to be of good intrinsic strength, well sited, and designed to cover the beaches to the North and East.

c. Development Area B. (fig. 119). (1) Shows the development of the position (B) as illustrated in figure 117. In its early stages the only really significant forward defenses were the pillboxes covering the length of the sea-front. Further back, a start had been made with the construction of defenses on the rising ground.

[Figure 119. Stages in the construction of a defense position.]
Figure 119. Stages in the construction of a defense position.

(2) At Stage 2, the position has taken its proper shape. It can be seen that the locality is sited in 2 'lines,' the first covering the sea-wall and its immediate sea approaches, while the second is set some distance back on rising ground having a good field of fire over the flat terrain intervening between it and the sea-wall.

(3) Another interesting feature which has appeared at this stage is the extensive ditch, presumably designed as an antitank obstacle, which has been dug behind the sea-wall. This ditch is 15 to 20 feet wide and some 600 yards long. It should also be noticed that the most southerly defenses, which first appeared only as a ring of wire with a few isolated weapon-pits, have developed into an extensive light antiaircraft position, with ground defenses incorporated. This position is almost certainly sited to fulfill a dual-purpose role.

6. THE DEFENSE OF A BEACH-LINE. a. General. (1) The system of siting defense positions as shown in fig. 120 of a coastal area in Burma is in evident contrast to that detailed in paragraphs 3 and 5. In this case, obvious emphasis has been laid upon siting defenses as far forward as possible. The frontal wire obstacles in places have been laid actually below the high water line, and the proximity of all the positions to this line is a very noticeable feature.

[Figure 120. A beach defense layout.]
Figure 120. A beach defense layout.

(2) Another interesting point of comparison between this layout and that of Attu Island is the extensive use made of wire obstacles and antitank ditches. In view of this, it is not unreasonable to presume that antitank mines have also been much more freely employed, probably both on the beaches and to cover the likely lines of approach of armored vehicles within the area.

(3) The relatively large number of strongpoints included within each defensive position is noteworthy. The majority of these are probably designed as fire positions for heavy and light machine guns, but some of the larger emplacements are designed to take heavier weapons, in particular the 37-mm antitank gun. This is a versatile weapon which has been freely used by the Japanese to fire against both landing craft and personnel, as well as in its original role.

(4) The siting of the central position, set back so as to enable it to cover by fire the rear of the forward positions, is of interest. It is presumably in this position, conveniently and centrally placed, that the local reserves would be held. Note also the similarity of this defense with the central fortified area described in Small Island Defense, Section IX.

b. Development of Area A. (1) The position shown in figure 121 is that marked as position (A) on the map of the beach defense layout (fig. 120). It is noteworthy for its speed of construction, since it reached the first stage shown in something under 3 weeks. During that time an extensive trench system was completed, 2 strongpoints were incorporated, and a long, forward zig-zag line of wire was erected. About a month later the locality had reached its final stage. By then, 4 new strongpoints had been constructed, a short length of communicating trench dug, and the wire defenses increased and brought round to link up with the creek on to which the position backs. A short forward length of brushwood also had been staked down in one sector.

[Figure 121. Position A on beach defense layout shown in figure 120.]
Figure 121. Position A on beach defense layout shown in figure 120.

(2) In general, this position may be said to be a typical example of the Japanese siting of a position for all around defense.

c. Development of Area B. (1) The defended locality illustrated in figure 122 (position (B) in the beach defense figure 120) was already at a fairly advanced stage of development when first photographed. Its chief interest lies in the number of strongpoints, eventually incorporated in it—no less than ten in a position with a total frontage of under half a mile. One of these strongpoints, added at a later stage as can be seen from the sketch, is noteworthy both for its exceptional size and for the fact that it is set right forward and not apparently connected with the general trench system.

[Figure 122. Position B of beach defense layout shown in figure 120.]
Figure 122. Position B of beach defense layout shown in figure 120.

(2) In general, the position seems to be one of particular strength, and it is to be noted that, in addition to being covered on one flank by the antitank ditch and brushwood obstacle of the adjacent position described below, it is also covered on the other flank by a creek.

d. Development of Area C. (1) The locality here given in detail in figure 123 is that shown as Position (C) in figure 120.

[Figure 123. Position C of beach defense layout shown in figure 120.]
Figure 123. Position C of beach defense layout shown in figure 120.

(2) When first photographed, the defended locality consisted of a fairly extensive trench system with numerous firing bays and one covered strongpoint together with a forward zig-zag line of wire. It can be seen that during the second stage of development the position was extensively altered by the filling-in of a large portion of the trench system and by the adding of 2 further strongpoints and a short length of trench. In addition, a road was brought in to the north of the position.

(3) During the final constructional stages the locality underwent even more radical changes, one of the most interesting of which was the addition of an antitank ditch formed by canalizing a small creek. For a distance of about 400 yards, its banks were dug vertical, thus making an antitank position. A considerable trench system was also added at one of the later stages, which, as annotated in the sketch, is remarkable for the size of the strongpoint incorporated in it as compared with those in the rest of the locality. This particular strongpoint is of the double-bay type, in contrast to the more common single-bay variety. The defenses of the position also were strengthened by the addition of another obstacle in the form of a staked-down line of bushes sited well forward on the beach. This obstacle extends for nearly a mile, covers the whole front of the position, and links up with the adjacent defended locality described above.

7. TYPICAL SMALL BEACH DEFENSE POSITION. In figure 124 there is illustrated in detail a beach defensive position which the Japanese constructed in Burma. The zig-zag trench system, within wire defenses, consisted of 5 groups, inclosing irregular areas, separated by a tidal creek. The trenches were approximately 5 feet wide at the top, with firing bays projecting outwards, thus affording an all round field of fire. Approximately 66 bays, equally divided between the two areas, were constructed in the system at intervals, varying between 23 and 70 feet. A pillbox, about 10 feet square, was located within the northern trench system covering the stream. Both double and single lines of wire, set on posts at approximately 10-foot intervals, furnished obstacle protection to the area.

[Figure 124. A typical beach defense position in Burma.]
Figure 124. A typical beach defense position in Burma.

8. DEFENSE INSTALLATIONS. a. General. (1) Japanese defense positions, both inland and coastal, generally include strong, mutually-supporting emplacements of a permanent or semi-permanent nature. The use of artificial obstacles has not been a universal feature. In various combat areas, however, there are indications that fairly widespread use is made of artificial obstacles for coast defense. This tendency can be expected to be intensified the more the Japanese Army is driven back on the defensive.

[Figure 125. Use of artificial obstacles in beach defense.]
Figure 125. Use of artificial obstacles in beach defense.

(2) Construction details illustrating obstacles have been taken from Japanese manuals and training instructions and may reasonably be expected to represent their current practice.

b. Obstacles. According to Japanese teaching, "obstacles are built to obstruct the enemy's advance and, combined with firepower, to destroy or hinder his movements, or to prevent surprise attack." They further state that wire entanglements and movable obstacles, combined with mines, abatis, and snares, normally are used. For antitank defense, ditches, mines, and obstacles for separating the infantry from their accompanying tanks are employed. The following details of their methods are given:

(1) Wire entanglements. (a) In figure 126 there is illustrated the net-type wire entanglement. Note that both barbed and smooth wires are used in the construction. Except for the lower horizontal line, the Japanese stipulate that the wires need not be tight, but they stress that the efficiency of the obstacle is increased by stretching wires between the main wires to thicken the net.

[Figure 126. A barbed-wire entanglement.]
Figure 126. A barbed-wire entanglement.

(b) The double-apron type illustrated in figure 127 was found at Betio Island in the Central Pacific. Lanes sometimes are left between the lines of this type so that movable obstacles may be utilized. Here, again, both barbed and smooth wire are utilized.

[Figure 127. A double-apron type of barbed-wire entanglement.]
Figure 127. A double-apron type of barbed-wire entanglement.

(c) Passageways through wire entanglements are illustrated in figure 128.

[Figure 128. Passage-ways through barbed-wire entanglements.]
Figure 128. Passage-ways through barbed-wire entanglements.

(d) In New Georgia the Japanese used a prickly vine in lieu of barbed wire to form obstacles around defensive positions.

(2) Movable barriers. The Japanese feel that movable obstacles are not very effective, but realize that they are easy to transport, set up, and conceal. They are used when an obstacle is needed to surprise opposing forces, when concealment of positions is necessary, when closing up a passageway in an obstacle, to block roads, and where it is difficult to drive posts in rocky or frozen ground. The length and height are varied according to the tactical requirements and the convenience of transportation. The following three figures illustrate the type commonly used:

(a) Movable wire barriers.

[Figure 129. Movable wire barriers.]
Figure 129.

(b) Cheval-de-frise (knife-rest) barrier.

[Figure 130. Cheval-de-frise (knife-rest) barrier.]
Figure 130.

(c) Spiral (concertina) barriers.

[Figure 131. Spiral (concertina) barriers.]
Figure 131.

(3) Other types of barriers. (a) Abatis. The Japanese recognize that abatis are easy to destroy, but they frequently use them in areas where trees are plentiful. Figure 132 shows a type made by cutting away the smaller branches of tree limbs, sharpening the ends of those remaining. The Japanese sometimes construct abatis by feeling trees at a height of 2 to 3 feet from the ground; the trees, not completely cut loose from their stumps, are felled in the direction of opposing forces, and their limbs are prepared much in the same manner shown above.

[Figure 132. Abatis.]
Figure 132.

(b) Folding screen. The frames are made separately and then connected as illustrated in figure 133. These may be used in rows.

[Figure 133. Folding screen.]
Figure 133.

(c) Wire snares. These may be of three types, illustrated in the following figures. It appears that either plain or barbed wire may be used, with the latter the most effective.

[Figure 134. Wire snares.]
Figure 134.

It is evident that snares are laid in "fields" at times on the principle of a mine field. In coast defense, they logically might be employed below the high water level, where they might well be expected to take attacking troops wading ashore by surprise.

(d) Example of the use of barbed wire. Barbed-wire defenses at Kiska were of four types:

Double apron (2 1/2-3 feet high at center and 10 feet wide).

Four strand fence (2 1/2-3 feet high—criss-crossed strands).

Low entanglements 1-1 1/2 feet off the beach designed merely to stall advancing troops momentarily.

Specially designed entanglements.

(1) Double apron fence predominated and was used principally to block the mouths of the stream valleys which at most beaches provide ingress to the interior. Usually, this fence was found erected along the slopes of grassy dunes just inland from the beach. In only a few cases was the wire established upon the beach itself. Pegs and posts used in stringing wire were of wood—apparently odds and ends left from building materials, etc.

(2) Four strand fence defenses were cleverly located at the crests of steep beach bluffs, where the barbs would retard the attacker after his energy had been spent in climbing. Heavy machine gun positions usually commanded the full length of these barriers.

(3) Low entanglements were stretched upon pegs just off the rocks of a few of the lesser beaches. In most of these cases the wire blended so well with rocks and sand that the observer could not see the barrier until he was within a few feet of it.

(4) (a) At Reynard Cove a unique wire defense was set up in which the barbed strands were strung from the top of a 15 foot bluff behind the beach to pegs driven in the beach itself. This arrangement created a thorny maze of wire through which attacking troops would have had to climb in order to reach the high ground behind the bluff.

(b) Barbed-wire defenses at Kiska were not set up in depth, however, and probably would not have hindered the attackers greatly. Except at 1 beach where 2 fences existed, no more than 1 wire barrier stood between the sea and the island's interior. At many of the beaches unused coils of barbed wire observed in storage piles may indicate that further defenses were planned.

c. Defense structures. (1) General. (a) Japanese defense structures have not followed a set pattern to date, but have been made to conform as nearly as possible to the surrounding terrain and to meet the immediate tactical requirements. With some exceptions, these structures have been relatively flat, extending 3 to 5 feet above the ground level, or irregularly shaped positions built around the bases of trees. A Japanese manual on field fortifications states that "it is most important not to adhere blindly to set forms in construction work, but to adapt such work to fit the tactical situation."

(b) When forced to take up an active defense, the Japanese apparently follow the theory that construction of defensive positions involves a continual process of development. First, the positions merely constitute a series of foxholes; subsequently, if time and circumstances permit, these are linked together to form a coordinated defense system. The third stage involves construction of strong points, bunker and pillbox types of earthworks, and log positions.

(c) Japanese positions have included bunkers, pillboxes, dugouts, shelters, blockhouses, rifle and machine-gun emplacements, fox holes, trenches, and antiaircraft emplacements and revetments. The terms "bunker," "pillbox," and "dugout" have been used fairly loosely, and it is sometimes difficult to do more than roughly differentiate between them.

(d) New Guinea. In New Guinea the Japanese terrain utilization between Buna Village and the coconut plantation at Cape Endaiadere was an excellent example of a complete defensive system. With the sea to their rear, they anchored their right flank on Buna Village where the unfordable Girua River and Entrance Creek enter the sea (figures 135, 136 and 137), and their left flank on the sea below Cape Endaiadere. They built a system of bunkers with connecting trenches on all the high ground; this forced the attacking force to advance frontally along rather narrow corridors of dry ground or through impassable swamps.

[Figure 135. Japanese defenses at Buna.]
Figure 135. Japanese defenses at Buna.

[Figure 136. Japanese defenses at Buna.]
Figure 136. Japanese defenses at Buna.

[Figure 137. Japanese defenses at Buna.]
Figure 137. Japanese defenses at Buna.

(2) Bunkers. (a) Generally speaking, bunkers may be said to differ from pillboxes by their size, shape, and shallow foundations. Usually they have been found on a large scale only in those areas where high-water levels preclude the digging of deep trenches, and in more or less open terrain (for example, in coconut groves and on the edges of airfields.)

Figure 138 illustrates typical bunker construction. The finished interior of bunkers varies from 4 to 6 feet in height, 6 to 10 feet in width, and 12 to 30 feet in length. The larger bunkers are found sometimes with 2 bays, or compartments, separated by a large solid block of earth. Each bunker has 1 or more narrow firing slits, difficult to hit even at close ranges. These slits are covered by some form of camouflage when not in use.

[Figure 138. A typical Japanese bunker.]
Figure 138. A typical Japanese bunker.

(c) In the Buna-Gona area, the bunkers and pillboxes (the latter have also been referred to as small bunkers) were built along the same general lines. With a shallow trench as a foundation, log columns and beams were erected, log revetment walls were constructed, and a ceiling then was made of several layers of logs, laid laterally to the trench. With the completion of this basic superstructure, the revetment walls were reinforced by such materials as sheets of iron, oil drums and ammunition boxes filled with sand, and additional piles of logs. Lastly, the outside was covered with dirt, rocks, coconuts, and short pieces of logs. For camouflage, the surface was planted with fast growing vegetation.

[Figure 139. Close-up of typical Japanese bunker.]
Figure 139. Close-up of typical Japanese bunker.

[Figure 140. Diagram of Japanese pillbox.]
Figure 140. Diagram of Japanese pillbox.

(d) Different types of entrances were used. Some had direct openings from fire trenches, while others had tunnels from the rear. With very few exceptions, all openings were constructed in such a way that the explosion of a grenade inside the entrance would not injure personnel inside the bunker.

(e) A few bunkers were used to shelter accompanying weapons such as antitank guns and therefore had large direct openings.

(3) Pillboxes. Usually Japanese pillboxes are constructed over, or near, dugouts, to which the enemy can flee for protection while being shelled or bombed. Some have been described as having front and rear compartments—the front part for firing and the rear for protection, storage of supplies, and rest or sleep. Some of the dugouts are 10 feet deep or more. Figure 141 is a front view of a typical pillbox. Note the narrow firing slit, cut at an angle to permit a wide field of fire, and the iron fasteners. Figure 142 shows how the inside of a large pillbox or shelter usually is constructed.

[Figure 141. Front view of a pillbox.]
Figure 141. Front view of a pillbox.

[Figure 142. Inside view of Japanese pillbox.]
Figure 142. Inside view of Japanese pillbox.

[Figure 143. Exterior view of pillbox.]
Figure 143. Exterior view of pillbox.

(a) Buna area. In the Buna area some of the pillboxes were made as follows: sand-filled oil or gasoline drums were placed at intervals in front of the trenches—enough interval was left to permit firing by automatic weapons and by rifles. Heavy palm logs were piled 3 to 5 feet in front of the drums, in such a way that they did not block the loopholes for firing. The structure then was covered with sod and otherwise camouflaged by shrubs and saplings, which were planted in a realistic manner.

(b) New Georgia. (1) Many of the pillboxes on New Georgia consisted of two decks, which permitted personnel to drop through a connecting door during heavy shelling. All were described as mutually supporting and very well concealed. The pillboxes usually housed heavy weapons, while communication trenches leading out on the flanks generally concealed light machine guns.

(2) Coral rock, better than ordinary rock because it is more resilient, formed part of the protective covering on many of the New Georgia pillboxes (see fig. 144). It was used in conjunction with coconut logs, earth, and miscellaneous materials at hand. A large number of the pillbox tops had as many as four layers of coconut logs which were topped with dirt and coral rock. Ferns and growing shrubs were planted in the chinks to round out a well-camouflaged appearance.

[Figure 144. A New Georgia pillbox.]
Figure 144. A New Georgia pillbox.

(c) Betio. Pillboxes—along with blockhouses, open and covered trenches, individual rifle emplacements, and open revetments—formed the main defensive system on Betio. They were situated within 100 feet of the high tide mark. The pillboxes were constructed mainly of reinforced concrete (several of these were 16 inches thick), coconut palm logs, and sand. Hexagonal (6-sided) steel pillboxes used as command posts, roughly in pyramidal shape, were found on all the beaches (see fig. 145). Apparently they had recently been installed, and were designed to be reinforced with concrete (concrete had already been placed around 2 of them). They had not been camouflaged, and were badly damaged, since most of them had not been reinforced by sandbags or coconut logs.

[Figure 145(a). A hexagonal steel pillbox.]

[Figure 145(b). A hexagonal steel pillbox.]
Figure 145. A hexagonal steel pillbox.

These pillboxes, apparently prefabricated, are designed to serve as command and observation posts. They have a double wall, between which sand and other material is placed for added protection. Apparently most of the beach-defense guns on Betio were emplaced in dugouts with overhead protection. Many of the dugouts were made of reinforced concrete. Figure 146 illustrates the concrete pillbox.

[Figure 146. A concrete pillbox.]
Figure 146. A concrete pillbox.

Ammunition and supply dumps were scattered about the island in bomb-proof dugouts.

(d) Burma. (1) In general, the Japanese pillboxes in the jungle country of Burma were found to be similar to those in the South Pacific. The two types of structures for bunkers commonly identified in coast defense positions in this area are shown in figure 147.

[Figure 147. Types of bunkers.]
Figure 147. Types of bunkers.

The details of construction are as follows:

(2) The Double Bay type is built in 2 sizes, 25 feet by 15 feet and 60 feet by 40 feet. They consist of mounds of earth from 5 feet to 12 feet in height, with a rear entrance well recessed into the mound. Forward, a central, apparently solid, block projects to form 2 bays. These bays vary in size.

(3) The Single Bay type consists of a roughly circular mound of earth about 25 feet in diameter and 5 feet high, with an entrance at the rear opening into a crawl trench or the main trench system. In front is a firing-slit; on, or slightly above, ground level, it is from 6 to 8 feet long and about 1 foot 6 inches to 2 feet high.

(4) The structures are designed for use by any of the weapons of the Japanese Infantry Regiment, but probably are best adapted for use by 37-mm antitank guns or heavy machine guns.

9. CONCLUSIONS. a. Pattern and principles. The tentative pattern and principles of coast defense which seem to emerge from study of the problems are: the Japanese may set their positions back from the coast line on high ground, with the intention of gaining complete control of ground and covering the beaches by fire alone, or if neighboring high ground is not available, their positions will be sited right at the water's edge with the intention of engaging any landing troops in direct combat at the moment most difficult for them. The selected localities will be well laid out and positions carefully constructed, while they will include a number of strong points with interrelated fields of fire. In comparison with inland defense positions, greater use is likely to be made of both anti-personnel and anti-vehicle obstacles, which may be of a variety of different types. Any artillery in the position will be boldly employed and sited, while its use in a dual-purpose role definitely is to be anticipated.

b. Tenacity. The Japanese conduct of the defense is characterized by tenacity and a determination to fight to the last man and the last round. As a corollary, any attacking force which gains a foothold in a Japanese coastal defense position must expect to meet concentrated and accurate fire from flanking strong points and must be ready to withstand an immediate and determined counterattack.

c. Weaknesses. The main weakness shown by the Japanese has been an inability to adapt themselves to the unexpected, and their coast defense positions have been found particularly vulnerable to surprise either in the nature or direction of attack. The Japanese also have shown themselves particularly susceptible to attack coming from ground higher than that on which their own positions were sited. Both these weaknesses were successfully exploited by the attacking forces on Attu Island.

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