Technical Manual, U.S. War Department, October 1, 1944
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Chapter VII: Tactics of the Japanese Army
Part II: Application of Tactics
Section IX: Small Island Defense
1. GENERAL. The information in the following section has been obtained mainly from observations made before and during operations against Japanese island bases in the Central Pacific Area and also from detailed studies of these bases after capture by Allied Forces. Studies of the Japanese island bases, made before their occupation by Allied Forces, checked very closely with the defenses as found on the ground. The defenses of these islands are probably typical of those of other Japanese island bases in the Pacific.
2. TYPES OF SMALL ISLANDS. a. General. The small islands of the Pacific fall into two main classes: the coral atolls and the volcanic islands. Japan has established bases on islands of both of these types. The particular type of base set up has been dictated by terrain considerations, geographical location, and strategical necessity. Thus, not all of the Japanese bases in the Pacific are large ones and not all of them can be considered as supply and command centers.
b. Coral atolls. Coral atolls are small low-lying, generally oval-shaped rings of islets inclosing a lagoon which may run from about 2 to 65 miles in diameter. (See figure 101.) These islets may extend from a few yards to a mile in width and from a few yards to several miles in length. The total land area of an atoll may range from a few hundred square yards to 6 square miles as in the case of Kwajalein, the largest known one. Atolls are rarely more than 25 feet above sea level, and, with few exceptions, are covered by dense growths of coconut palm, pandanus, and salt marsh. The water table is usually only a few feet below the surface of the ground, and, as a result of this, deep entrenchments and fortifications cannot be dug. A bomb crater generally fills up with water in a few hours. This limitation on excavations on coral atolls forces the defenders to build their fortifications above the ground in most cases and has resulted in the adoption of the bunker and pillbox type of fortification for atoll defense. The one exception to the above mentioned limitation is the antitank ditch. When these are constructed in low lying places they sometimes fill up with water and become a better obstacle. Some atolls have excellent large vessel anchorages in the lagoon (Kwajalein and Wotje), while others have no channel into the lagoon and only provide offshore anchorage, such as Uterik and Namorik.
c. Volcanic and raised coral islands. The Marshalls and Gilberts, and all but five of the Carolines, are coral islands. The exceptions are volcanic, or volcanic and coral. Some of the coral islands, such as Nauru and Ocean, are not atolls and contain no lagoons. These islands are large, circular, raised land masses and have been developed into bases by the Japanese. In common with the atolls they are surrounded by coral reefs.
d. Volcanic Islands. The volcanic islands of the Central Pacific have been developed into major bases by the Japanese. These run in a line from East to West as follows: Kusaie, Ponape, Truk, Yap and Palau. (See fig. 102.) These volcanic islands are much larger in size than the atolls and can support considerable garrisons. Most of the volcanoes are extinct, but some in the northern Marianas are active. Some, like Truk, are groups of islands. Truk is completely surrounded by a coral barrier reef located some distance off shore; and it contains a fine harbor and considerable anchorage space. Fresh water supplies are generally ample, and there is room on these islands for air-bases. The terrain is rugged and mountainous and is similar to some of the Hawaiian islands.
3. DEFENSE OF VOLCANIC ISLANDS. The defense of volcanic islands consists of beach positions, heavy naval guns up to 12-inch size, and mobile reserves. Beach defenses consist of observation posts, strong points, and obstacles, but these are not to be considered a perimeter defense. Large-sized units are held as reserves and are employed in counterattacking at threatened points. The defenders have the advantage of dominant observation, knowledge of the terrain, and large amounts of supplies. In addition to this, they have maneuver room, and if driven into the hills are quite capable of carrying out harrassing operations for long periods. Airstrips are located on the volcanic island bases, and both land-based and naval aircraft are used in the defense. Antiaircraft artillery is used in the defense of harbors and landing fields. Army troops, as well as Special Naval Landing Forces, are likely to be encountered on these bases.
4. TYPES OF ATOLL ISLAND BASES. Some of the atolls have been developed into large airfields, while others are not large enough to contain an airstrip, and are used as seaplane bases where the aircraft use the lagoon as an anchorage and landing area. Others are not suitable for either of the above types of base and are used as weather stations and radio relay points.
5. LIMITATIONS ON SIZE OF GARRISONS. There is a physical limitation on the number of men that can be placed on an atoll in view of the limited space and the difficulties of fresh water supply. On most atolls, evaporators must be used to supplement the rainwater cisterns and the brackish water wells. The problem of supply and storage area also limits the size of garrisons. Natural food supplies on an atoll are very scanty, with the exception of fish, and since this is a staple of Japanese diet it is used a great deal to supplement the rations shipped in. Refrigeration is also a problem as foodstuffs spoil rapidly in tropical climates.
6. CORAL REEFS. All coral atolls are surrounded by reefs on both the ocean and lagoon sides of the islets. This is a great natural obstacle in favor of the defenders. In the Makin and Tarawa operations, certain landing craft could not negotiate the reef, and the landings therefore had to be made in amphibious tractors or by wading ashore several hundred yards in chest deep water. These reefs may extend from shore for a few yards or as much as a half mile. During high tide the reef is submerged below a few feet of water, while at low tide it may be completely exposed and above water. At low tide the reef makes a perfectly flat cleared field of fire for the defenders. As a rule no reef is found opposite the mouth of a river.
7. TYPES OF ATOLL DEFENSES. Two major types of defense are encountered on coral atolls: the perimeter defense and the fortified central area with outposts.
a. Perimeter defense. The perimeter defense is the one almost always used by the Japanese and is usually encountered on small islets in the atoll ring which are generally large enough only for an airbase. The landing field covers most of the islet, and the remainder of the installations are located around the field. Defensive works are a continuous band around the islet and are not generally in great depth. Practically all men are committed to the perimeter at the outset of a fight with few troops held in reserve. All types of weapons are on the perimeter, from 6.5-mm rifles to 8-inch naval turret guns. Trenches, pillboxes, barbed wire, mine fields, and antitank ditches are used in the defense. An example of the perimeter defense is shown in figure 103.
b. Fortified central area. Another type of defense is used on islets which are long and narrow, or which are too long for the use of a perimeter defense with the amount of troops available at the time. The Japanese do not fortify the entire perimeter, but generally group their installations in a central area, build tank traps at the ends of the area, and install their defensive works behind the tank traps facing out. Small outposts are placed on the remainder of the islet, but there is no continuous line of defenses outside of those in the fortified area. The tank ditch apparently plays the most important role in the defense plan, since a great part of the defenses are arranged to keep attackers from penetrating that line, and the majority of the pillboxes are located there. The heavier guns are placed near the shores between the tank ditches, since most of them are fixed guns and could not be used for close-in defense. The outposts generally consist of antiaircraft batteries and machine gun posts located at the extremities of the islets. A typical central fortified area is shown in figure 104.
8. DETAILS OF ATOLL DEFENSES. a. Antiaircraft defenses. (1) Early warning measures. The first evidence of Japanese radar in the Central Pacific was encountered early in 1943. While the performance of this equipment has not been up to accepted standards, improvement can be expected in the future. The extreme range of Japanese radar noted to date is 60-70 miles. The Japanese also make use of watchers as well as radio and weather stations on small undefended atolls as a means of passing early warning back to their defended bases.
(2) Antiaircraft weapons. In small islands encountered so far the only heavy antiaircraft guns definitely identified are the 75-mm (7-cm) antiaircraft gun and the twin-mount 127-mm dual purpose gun. These weapons are generally in two or three gun batteries and seldom in large groups. All weapons are well emplaced in heavily constructed revetments. The Japanese also have 20-mm and 25-mm antiaircraft automatic weapons. In the Gilberts 13.2-mm machine guns in single and twin-mounts were used. Automatic 7.7-mm and 6.5-mm guns are also used as antiaircraft weapons.
(3) Passive defensive measures. The Japanese have constructed heavy air raid shelters of palm logs, sandbags, and loose sand. On Makin and Tarawa these were extensively used during the bombardment by the Japanese troops, civilian laborers, and natives. Fox holes (weapon pits) and heavily revetted buildings also were encountered. Storage buildings and planes were dispersed as widely as the size of the islets permitted.
(4) Camouflage. Where used, camouflage is excellent, but is confined mostly to small weapons emplacements, antiaircraft guns, and alternate positions. Some buildings are dazzle painted. Great use is made of dummy installations in island defense and the Japanese take great pains in this. Most camouflage is obtained by the use of natural materials. (See figs. 105, 106, and 107.)
b. Antilanding measures. (1) Obstacles and mine fields. The greatest obstacle in the attack of a small island is the coral reef which surrounds it. The reefs are found on both lagoon and ocean sides of the atolls and on the ocean side of other islands. However, there may be small stretches of beach on the lagoon side of the islets which are free of reefs and small stretches of reef-free beaches on the windward face of the ocean side of the islets. The Japanese have strengthened this natural obstacle by placing concrete pyramids (tetrahedrons), horned scullies, coral cairns, barbed wire, and log boat barricades on the reef. (See figs. 108, 109, 110.)
Interspersed with these is a mine field of waterproof mines containing a heavy charge of explosives detonated by a chemical-electric fuse. The detonating nipples of the mines are connected by wire to the obstacles and some also by wire to the shore. They can then be detonated by three methods: by direct contact, by hitting a trip wire, or by a pull wire from shore. Antitank mine fields also are placed on the beaches on both flanks of the antitank ditches. The reef is hard and will support medium tanks with ease.
(2) Pattern of fire and defensive weapons. (a) The defense of the beach is built around the machine gun and a final protective line. The beach defense usually consists of a shallow line of strong points, with a secondary line of lesser density located slightly to the rear. Because of the small size of the islands, depth of the defense is limited. The strong points consist of a group of bunkers and pillboxes connected by trenches and are covered by fire from riflemen in fox holes all around the installation. These fox holes are connected by communication trenches. The pillboxes and bunkers within the strong point are mutually supporting. Each rifleman protecting the pillbox generally has several alternate positions. He runs from one to another, and during combat this practice sometimes leads to an over-estimation of the strength of the defenders. The Japanese make extensive use of hand grenades in the defense of small islands, and flame throwers were present on Tarawa and Kiska.
(b) The Japanese defense of small islands is based on breaking up an attack before it reaches the shore, and all coast defense guns up to the 8-inch are sited so that they can be employed against small boats and landing vehicles. Batteries have local fire director control, generally 2 or 3 guns with observation towers at the gun position. Flat trajectory weapons predominate, with howitzer type weapons in the minority. These weapons are placed well forward on the beach where direct laying can be used against landing craft. Their grouping is shallow, all weapons being sited so that they can be used to repel a sea-borne invasion. This also applies to antiaircraft guns, the secondary mission of which is to repel surface craft and landing vehicles. To date coast defense guns found on these islands have ranged from 3 to 8 inches. The 8-inch guns were in turrets, while the 6- and 5-inch had shields. All were well emplaced in heavy revetments, with ammunition storage in covered emplacements close to the guns.
(c) All machine guns are sited to fire outward around the perimeter. Most of them are sited for cross-fire and cover the beach obstacles with enfilading fire. A few are also sited to fire to the rear of their positions. Some machine guns are in open emplacements, while others are set up in pillboxes. Those in open emplacements are generally dual purpose and have wide fields of fire; those in pillboxes have narrow fields of fire and are sited to fire only in one direction. All emplacements are protected by riflemen in foxholes and trenches around the installations. Some infantry regimental guns and battalion howitzers are used. Small use was made of trench mortars at Makin and Tarawa; however, this probably was so because of the limited number of such weapons in the Special Naval Landing Forces. Small arms used are both 7.7-mm and 6.5-mm caliber.
(3) Field fortifications. The Japanese defense of small islands is characterized by the extensive use of field fortifications. The bunkers and pillbox emplaced machine guns are the backbone of defensive fire. These fortifications have been developed from small installations, composed of a single layer of palm logs and sand bags and large enough for only a few men, into massive structures 6 to 8 feet thick, housing more than a squad. Palm logs are giving way to reinforced concrete and completely enclosed steel structures. These bunkers are relatively safe from damage by anything less than a direct hit by a 155-mm shell or larger. (See figs. 111-112.) The Japanese defend these to the last man, and some have been known to remain in these positions for days amid rotting corpses subsequently to come out fighting during mopping-up operations. (This was true at Tarawa and Makin.)
A typical "bunker" and pillbox strong point protecting a coast defense battery is shown in figure 113.
c. Antitank defense. (1) Obstacles. To date the Japanese have constructed tank barriers only on the islets where there are airstrips and seaplane bases. These, generally, are antitank ditches and lines of obstacles. The ditches run from 4 to 8 feet in depth and from 10 to 20 feet in width. (See fig. 114.) They extend completely across the islet in the case of the central fortified zone and usually are protected by mine fields on the flanks. At communication points along these ditches the Japanese construct bridges guarded by palm log barricades filled with rocks. In the perimeter defense these ditches are found at the ends of airstrips and backing up critical landing beaches. Low palm log hurdles are placed in front of the ditches in order to slow up tanks approaching the trap. Concrete pyramids (tetrahedrons) and horned scullies are used on the reef as continuous barriers. Only very recently have the Japanese begun to use mines in defense of islands. On Tarawa a thick mine field was laid on the reef in the space between the tetrahedrons. On Kiska thin mine fields were found. On Makin several hundred mines were found in a warehouse, but none had been laid. Indications point to greater use of mines in the future.
(2) Antitank weapons. Although they possess more modern guns the Japanese so far have employed 37-mm rapid-fire guns in island defense. These guns are employed behind the tank barricades and ditches and are usually in open emplacements. Alternate positions are prepared for these guns, since the Japanese tactics are to keep antitank weapons mobile. Light tanks were included in the defense of Japanese bases recently captured, and the Japanese doctrine was to employ them against tanks landed by the attackers. All Japanese small arms are assigned the mission of firing against tanks and have done so in Pacific operations. Another antitank weapon is the magnetic mine which has been in use in all Pacific theaters.
d. Supply installations. (1) Japanese supply installations on small islands closely follow the principal of dispersion. Supplies are placed in small dumps, in many cases in shallow excavations, as a protection against bombing. Perishables and food are generally in warehouses, and refrigeration is used on some bases. With the increase in the size of island garrisons which has been apparent since the latter half of 1943, the Japanese have been placing a large part of their supplies on islets adjoining those on which their major defenses are sited. In the case of ammunition, the warehouses are connected by a narrow gauge railway. Causeways have been built between the islets to make communication easier.
(2) The Japanese also secrete small arms and ammunition in caches at various points on the islets, as at Makin, so that in case of a withdrawal from their fortifications they can fall back along their supply line. Canteen supplies, beer, and sake were plentiful, and there was no shortage of food. Supply vessels called at regular intervals. From most data available a level of supplies of about 6 months seems to have been maintained at all out-lying bases. Clothing supply was good. The Japanese in the Gilberts and Marshalls were all well supplied with food and clothing and were in excellent physical condition.
(3) The policy of reinforcing the island bases has been that each ship calling at a base drops off a few more guns and additional ammunition. The caliber varies, but each type of weapon is incorporated in the defense according to its capabilities. Ships bring supplies from central island bases or direct from the Japanese mainland.
9. AIRFIELDS AND SEAPLANE BASES. a. Types of airstrips. The type of airstrip used on the Japanese bases depends on the shape and size of the islet. On the circular islets the field is made up of two crossed strips. On crescent-shaped islets a triangular-shaped pattern is used. On narrow islets a single strip is laid. These patterns are fairly regular. Runways are surfaced with coral and have large turn-arounds at each end. The layout of the runways takes advantage of prevailing winds. Aircraft revetments are the palm log type, covered with sand. A new development has been the use of small railway cars loaded with logs and sandbags which, during an air raid, are rolled in place in front of revetments to serve as blast walls. Hangars are located on the service aprons and are of conventional design. The presence of underground hangars has been indicated on some bases. Seaplane bases are uniform in design, and generally consist of two concrete ramps extending into the lagoon, with a concrete apron on the shore bordered by hangars and machine shops. As a rule these seaplane bases are in the center of a fortified area.
b. Type of planes used. The types of planes encountered on small islands have been: four-engined flying boats; twin-engined medium bombers; land based pursuit; and single-engined float planes. The four-engined flying boat is used for long-range patrol work and now carries the brunt of aerial reconnaissance. Japanese interception tactics are normal.
10. SUCCESSIVE STAGES IN CONSTRUCTION OF BASES. Since the Allied offensive in the Central Pacific began, construction of air bases on most atolls long enough to support an airstrip has been noted. The first stage in construction of a base is building the airstrip. The antiaircraft defenses are next set up followed by emplacement of coast defense guns. Tank barriers and ditches come later, and pillboxes and bunkers last. The Japanese have used considerable numbers of civilian laborers in the construction of their bases. These laborers are not permitted to work on the defenses but only on the airstrips, barracks, and docks. They are carefully segregated from the Japanese military personnel and are not permitted access to the fortified zone. During the defense of some bases, they were given arms by the Japanese and were told to fight. Many did, while others merely hid in air raid shelters and then surrendered.
11. HISTORICAL EXAMPLE OF JAPANESE DEFENSE AGAINST AN AMPHIBIOUS ATTACK ON A CORAL ATOLL. a. Defense of Makin Atoll. (1) During assault phase. The Japanese used air attacks in order to break up the Allied assault before it reached the island. As the transport groups moved in for the landings the Japanese opened fire with every weapon available. During the initial landing small arms and automatic weapons opened up when the attacking forces came within range. They did not wait until troops got close in. Practically all of the defending troops were in the defensive works, and very few were held in reserve. They were unable to destroy or break up the assault or to prevent the establishment of beachheads. In the early stages Japanese defense was mainly by fire and very little by movement. Many remained in their bunkers and pillboxes until killed. During the first night the Japanese tried to infiltrate into the attackers' lines to reoccupy positions lost during the day's fighting.
(2) During mopping up phase. When their line of bunkers and pillboxes was breached, the remaining Japanese broke up into small groups and withdrew into covered areas, where they re-formed and then counterattacked. When these counterattacks were broken up, they formed into small groups to take refuge in bunkers, air raid shelters, and the salt marsh jungle. Japanese suicides were frequent during the mopping-up phase. Some remnants of their forces hid out for days after the fighting ceased, subsisting on coconuts and stolen rations, while others tried to escape out to sea. Probably because of the speed of Allied operations and the rapidity with which their positions were overrun, the Japanese were unable to destroy equipment and stores.
(3) Conclusions. Since the Japanese were unable to break up the Allied assault before it hit the beach and prevent landings, their beach strong points were overrun rapidly, and their defense plan was nullified. In order to counterattack they had to withdraw men from their prepared positions and thus lost the advantage of their fortifications. Their prompt counterattack was entirely in keeping with their normal tactical doctrine.
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