1. GENERAL

The following information about the Japanese defense of Betio Island is based on a joint study made by Marine Corps, Army, and Navy intelligence personnel.1 The fundamental points brought out by this study are presented in the Intelligence Bulletin so that they may be widely disseminated, particularly among junior officers and enlisted men of the Army.

That the Japanese organized their Betio defenses for an all-around, decisive struggle to keep United States forces from reaching the beaches is borne out by a study of the map on pages 34 and 35. This map, drawn to scale, is designed to show an over-all picture of the enemy defensive setup. The insets show important areas in detail.

The Japanese beach defenses consisted of well-emplaced and well-sited weapons, various types of obstacles, and mines. The weapons included grenades, mortars, rifles, light and heavy machine guns, 13-mm dual-purpose machine guns, 37-mm guns, 70-mm infantry guns, 75-mm mountain guns (Model 41), 75-mm dual-purpose guns (Model 88), 80-mm antiboat guns, 127-mm twin-mount, dual-purpose guns, 140-mm coast-defense guns, and 8-inch coast-defense guns.

The obstacles included pyramid-shaped, reinforced concrete obstacles, which were placed about halfway around the island on the coral reef; an antiboat barricade, made of coconut-palm logs: double-apron barbed wire; a perimeter barricade, constructed chiefly of coconut-palm logs; and antitank ditches, dug a short distance back of the perimeter barricade.

Antipersonnel and antiboat mines were laid on the fringing reef—frequently between the concrete obstacles—and on the beaches.

Several Japanese flame throwers, were found.

Appropriate fire-control equipment was installed for the coast-defense and antiaircraft batteries, including range finders, directors, and searchlights. The weapons, as a rule, were mounted in strongly constructed emplacements made of coconut-palm logs, reinforced concrete, and revetted sand.

The Japanese employed the 13-mm (approximately .50 caliber) machine gun as their basic beach-defense weapon along the entire north coast and on both sides of the eastern tip. Along the western and southwestern coasts, the 7.7-mm heavy machine gun was the basic weapon.

The organization for defense inshore was haphazard. Beaten on the beaches, the Japanese fell back to bomb-proof ammunition shelters and personnel shelters inshore from the beaches, and fired from the doors of the shelters. These were blind to attack from several directions; they were not designed as blockhouses, and had only a few firing ports.

2. OBSTACLES

a. Reinforced Concrete Pyramids (tetrahedrons)

Almost up to the time United States Marine forces landed on Betio, the Japanese were working rapidly to surround the island with pyramid-shaped, reinforced concrete obstacles on the fringing reef. The process was approximately half completed at the time of landing. Figure 1 shows how the enemy had been molding these obstacles on the beaches. Before concrete was poured into the inverted forms, the angle irons which formed the corners and horns were driven into the ground.

The actual size of the obstacles varied. Their bases usually were about 4 feet wide and their height was determined in accordance with the average height of the water over the reef. The center-to-center distance between the obstacles varied from 6 to 20 feet, on different beaches.

Located about midway out on the reef, these obstructions were just high enough to break the water at high tide. They were designed to obstruct landing boats or to canalize them into predetermined areas which could be swept by the fire of antiboat guns, ranging from 13-mm machine guns to twin-tubed 127-mm guns.

In addition to the reinforced concrete obstacles, the Japanese rounded out their reef defenses by using wire, barricades, mines, and fairly large piles of coral rocks, which in some areas were staggered among the pyramids and strewn about the reef. The rocks were from 6 to 18 inches in diameter, and the piles were 4 to 6 feet high.

b. Wire

The Japanese used three types of wire obstacles: high double-apron fences, low double-apron fences, and single-apron fences.

The high double-apron wire was placed inshore of the pyramidal obstacles and below the high tide mark. It was constructed to canalize assaulting troops into direct enfilade fire from emplaced light and heavy machine guns and into trip wire.

Immediately inshore from the beach, high double-apron wire was placed directly in front of tank ditches, and low double-apron wire was placed behind the ditches.

Low double-apron wire was placed above the high-tide mark, and was employed to canalize or obstruct assaulting troops in front of covered machine-gun emplacements.

Single-apron wire was erected in front of some portions of the log barricades. The wire was strung from the tops of vertical logs to stakes in the sand forward of the barricade.

c. Antiboat Barricade

On the south beach of Betio, the Japanese constructed an antiboat barricade with coconut-palm logs (see fig. 2). The barricade, which was 10 feet high, was shaped like a wide V, with one leg 700 yards long and the other 300 yards long. The logs were secured in place with wire and soft steel fasteners, 1/2 inch in diameter.

The barricade was planned to divert landing boats to the east and west of the center of the beach, and into areas which would receive flanking fire from emplaced heavy machine guns, dual-purpose guns, and heavy antiboat guns.

d. Perimeter Barricade

The Japanese constructed a barricade of coconut logs around virtually the entire perimeter of the island (see fig. 3). There were a few unbarricaded spaces, but these were protected by antitank ditches. Many of the perimeter-defense emplacements, because of the nature of their construction, were in themselves good barricades. (These are described later.)

Each coconut-log barricade followed one of three general designs: (1) Log barricade with built-in rifle or light machine-gun emplacements (see fig. 4); (2) log barricade without built-in weapon emplacements (see fig. 3); and (3) tree stumps laid with their bottoms facing the sea.

That portion of a barricade which did not have built-in and covered rifle or light machine-gun emplacements had, instead, open sandbagged emplacements behind the barricade for these weapons.

e. Antitank Ditches

Antitank ditches on Betio were only 5 to 7 feet deep—because the water table was about 8 to 10 feet below the surface—and were 12 to 14 feet wide. Only one ditch was revetted—with coconut-palm logs on the opposite side of the expected tank approach.

Wire obstacles were placed on a road at points where antitank ditches formed a junction. Covered machine-gun emplacements were sited at the ends of some antitank ditches. Apparently antitank-gun fire was to be supplied by 37-mm, 70-mm, and 75-mm guns.

On the south beach, one antitank ditch was located about 10 yards inshore from a line of coconut-palm stumps. Machine guns emplaced under cover and 37-mm guns emplaced without overhead protection were located inland to provide direct covering fire for the antitank ditch. At other points along the south beach, short sections of antitank ditches took the place of barricades.

3. MINES

Four types of mines were used by the Japanese on Betio: Model 93 antivehicle mine, Model 99 armor-piercing mine (magnetized), an antitank mine, and an antiboat mine.

The Model 93 was used primarily against personnel. This mine is usually placed in patterns of diagonal rows, with the mines about 30 inches apart. The brass plug on top of each mine is generally at ground level. The device is activated by pressure on the plug.

The Model 99 (magnetic) mine was used primarily against tanks and armored cars.

The antiboat mine was found in large numbers on the south and west coasts. It is believed that the Japanese were in the process of encircling the island with these mines at the time the United States Marines landed. Along the south beach the mines were located between the double-apron wire and the shore. Laid in a double lane paralleling the beach line, they were placed about 20 yards apart. In some places the mines were covered by 2 to 3 feet of water at high tide, while in others they were dry both at high and low tide.

Only a few of the antitank mines were found. These were buried in the sand, with their necks exposed.

4. GRENADE DISCHARGERS AND GRENADES

The Japanese used Model 89 grenade dischargers and two types of hand grenades against landing boats and personnel. The grenade dischargers apparently were assigned by sections throughout the beach-defense system. In addition, several sections were placed behind antitank ditches in the eastern portion of the island.

5. RIFLE AND LIGHT MACHINE-GUN EMPLACEMENTS

a. General

Light machine-gun emplacements and rifle emplacements (or pits) were constructed to accommodate these weapons interchangeably. Most of the emplacements were located directly in the beach barricade. Some of these were protected by a strong cover, while others were open. Located close to the beach barricade were several light machine-gun emplacements made of concrete.

b. Covered Type

The covered emplacements in the beach barricade were topped with at least one layer of coconut-palm logs about 10 inches in diameter and with loose sand and coral at least 1 foot thick (see fig. 4). Inside, these emplacements were 4 feet long, 4 feet wide, and 4 feet high. The occupant or occupants were protected by a thick blast wall, located so close behind the emplacements that entrance was difficult (see fig. 5).

Each of these emplacements had a small firing port, 12 inches long and 6 inches wide, which permitted little more than frontal fire. Inside, under the firing port, there was a small fire step or shelf on which a light machine gun could have been placed.

Some of the covered rifle emplacements connected at the rear with communication trenches, which were revetted with vertical logs, boards, wire drums, and corrugated sheet steel. Some of these trenches were covered.

c. Open Type

The open emplacements in the barricade system were mere slots behind the log wall, protected on the sides by logs bracing the barricade, by sand, and by a board revetment (see fig. 6). Only rifles and light machine guns could be fired from these emplacements.

Some of the open-type emplacements were located a few feet behind, and above the top of, the barricade. These emplacements were constructed of sand and were revetted with corrugated sheet steel and logs.

d. Concrete-pillbox Type

Figure 7 shows a light machine-gun emplacement made of reinforced concrete and provided with three firing ports. Most of these emplacements were built just forward of the beach barricade, and were sited so as to provide frontal fire to cover tactical wire, and flanking fire to cover the front of the barricade. Since other emplacements in the vicinity of the concrete pillboxes provided flanking and frontal fire, the pillboxes may have been designed primarily for firing on targets of opportunity.

The tops and sides of the concrete emplacements had an average thickness of 14 inches; they were reinforced by steel rods 1/2 inch in diameter.

Although these emplacements were independent of the beach barricade emplacements, they were connected with the barricade by tunnels, communication trenches, and blast bays made of logs and concrete.

e. Shields for Riflemen

Figure 8 shows details of two types of shields for riflemen. The shields, found near the main runway of the Betio airfield, are light and easy to carry; therefore, in effect, they are mobile rifle emplacements which afford some protection. They are only 1/8 inch thick, but are made of hardened steel. They were painted a dull brown.

6. HEAVY MACHINE-GUN EMPLACEMENTS

a. Built into Beach Barricade

Most of these were encountered in the strongly organized and well-constructed barricade system on the west and southwest coasts. Within this system, the heavy machine-gun emplacements were spaced at fairly regular intervals, and were separated by covered rifle and light machine-gun emplacements, which afforded local protection.

Roughly, the built-in emplacements were of two types: those with a single firing port, for frontal fire, and those with two firing ports, for flanking fire. Both types were made of logs and sand, and were an integral part of the beach barricade.

The single-port type provided frontal fire over the reef and covered the approaches to machine-gun emplacements which were sited for flanking fire.

Most of the single-port types were well designed and constructed (see fig. 9). The sides and top consisted of two or three layers of coconut logs and a layer or two of sandbags covered with sand and coral. The sides were banked with sand to add protection and camouflage.

The outside width and length of these emplacements were 18 to 20 feet. Inside, they were divided into compartments, one or more of which were used to store and protect ammunition—these were always in the rear.

Most of these emplacements were connected with bomb shelters, rifle and light machine-gun emplacements, command posts, and ammunition dumps by means of revetted communication trenches.

The two-port type of heavy machine-gun emplacement could mount two heavy machine guns (7.7-mm) and could possibly mount 13-mm machine guns. The firing ports in nearly all cases were sited for flanking fire along the tactical wire entanglements and boat obstacles. In fact, the design of the emplacements prohibited frontal fire.

The top and sides consisted of two, and sometimes three, coconut logs, which were covered by two layers of sandbags and rounded off at the sides with sand and coral. The finished structure, except for the entrance and connecting communication trenches, appeared from the top as a large sand mound within the barricade.

The outside length and width of these emplacements measured approximately 24 feet. Inside, they were divided into compartments to give added protection to both personnel and ammunition. The latter was kept in a separate, well-constructed room in the rear of the emplacements (see fig. 10).

A second type of twin-port heavy machine-gun emplacement (or casemate) was made of concrete. Only two of these were found. Each had two ports, for flanking fire, and each had two adjoining antiaircraft emplacements. Although this type of emplacement apparently was designed for the use of 13-mm machine guns, indications are that 7.7-mm machine guns were used in the two found on Betio.

b. Outside of Beach Barricade

Open-type heavy machine-gun emplacements in the interior of the island consisted of simple circular pits dug into the ground and revetted with boards, corrugated sheet steel, and sandbags (see fig. 11). Logs and oil-drum pedestals were used as mounts for guns. These emplacements obviously were designed primarily for antiaircraft fire (probably 7.7-mm machine guns). They were located at irregular intervals around the airfield and inshore of the barricades.

Apparently the Japanese did not construct any single-port, covered heavy machine-gun positions inshore of the beach barricades. However, as the situation developed, the enemy made full use of the entrances to bomb-proof shelters as machine-gun firing positions.

7. 13-MM MACHINE-GUN EMPLACEMENTS

a. General

In defense of Betio, the Japanese employed both single- and twin-mount 13-mm dual-purpose machine guns. All were pedestal-mounted, and were located in open emplacements to permit antiaircraft fire, as well as fire on ground troops. Most of these guns were sited so that frontal or flanking fire could be placed on the beach and reef. Several were located on top of high structures, such as the magazines near the 127-mm dual-purpose guns.

b. Single Mount

Emplacements for the single-mount gun were approximately 4 feet deep. The gun pedestals generally projected almost to the ground level; thus the gun itself was only 1 to 1 1/2 feet higher (see fig. 12).

In construction, these emplacements varied as follows:

(1) Six-sided emplacements, 10 feet in diameter, with the retaining walls consisting of a layer of horizontally placed logs, or boards and logs, or sandbags, and a sandbag parapet. In many of these emplacements, the parapets were two or three sandbags higher than others—presumably for protection against blast and fire. In at least one position, communication trenches led to the emplacements.

(2) Ten-foot-square emplacements, with log and sand-constructed shelters 6 1/2 feet high and 10 to 15 feet wide. These emplacements had coconut-log retaining walls which were banked with sand.

(3) Square or six-sided emplacements, raised about 3 feet above ground level. Walls (2 feet thick) of sand and sandbags were contained by corrugated sheet iron, posts, and so forth. The inside diameter was 10 feet; the exterior diameter, 14 to 15 feet. (One gun was provided with a 3/8-inch-thick steel shield.)

(4) Twin, eight-sided concrete emplacements, which were part of a ground machine-gun and antiaircraft machine-gun casemate (already described in par. 6a).

(5) Emplacements on top of buildings and revetments; these were circular and simply constructed of sandbags.

c. Twin Mount

Two types of emplacements for the twin-mount guns were noted:

(1) Slightly pear-shaped emplacements, 10 feet across at the rear and slightly less at the front; 3 feet high at the rear and sloped forward to a height of 2 feet at the front; walls made of sandbags 3 feet thick, and further banked by soft sand.

(2) Square emplacements atop ammunition magazine; 10 feet across on the inside; walled by boards and banked with sandbags and loose sand to a thickness of 3 to 4 feet; 2 feet high.

Figure 13 shows one of the three known types of pedestals for the twin-mount machine gun.

8. FIELD GUNS AND EMPLACEMENTS

The location of emplacements for the three types of field guns used by the Japanese on Betio indicates that their primary mission was antiboat defense. Some of these guns—75-mm mountain gun (Model 41), 70-mm battalion gun (Model 92), and 37-mm rapid-fire gun (Model 94)—were used as support weapons after our beachheads had been established.

Emplacements for all three guns were roughly of the same type. The interior was arrow-shaped or, in a few cases, egg-shaped, with the entrance in the broad end and the firing port in the narrow end (see fig. 14). The walls consisted of a single layer of logs, laid either horizontally or vertically. In some cases they were lashed to retaining posts or were joined at the corners with steel rod fasteners. The walls were banked outside with mounds of sand, which were 4 to 5 feet in width at the base.

Most of the emplacements were roofed with a single layer of logs, covered to a depth of 2 to 2 1/2 feet by loose sand. A few emplacements had no roofs (see fig. 15).

All emplacements had an unroofed entrance passageway in the rear, 5 to 6 feet wide and 5 to 10 feet long. In most cases the passageway was curved so that a second rear wall could protect the rear of the emplacement from blast or fire.

The firing ports of all emplacements except those on the northwest coast were sheltered from blast and small-arms flanking fire by means of log- and sand-banked wings. These were 5 to 8 feet long and opened at an angle of about 100°. Some emplacements on the beach were protected by low double-apron wire.

These emplacements, although obviously not designed for small-caliber weapons, in many cases were used as last-ditch positions for riflemen and machine gunners. In a few instances, the guns were removed from the emplacements, turned around, and fired inland from the rear.

9. DUAL-PURPOSE GUN EMPLACEMENTS

a. For 75-mm, Model 88, AA Guns

Eight of these guns were emplaced on Betio. Four were sited as a battery near the northwest corner of the island, and the other four were arranged in pairs, one each on the north and south coasts of the eastern tip.

Except for minor variations, all emplacements for the Model 88 were similar. Dug to a depth of about 5 feet below the ground level, they had five sides, which were revetted with empty oil drums and with 1-inch-thick boards secured by vertical coconut logs or by solid coconut-log walls. Ammunition ready-boxes of varying sizes were built into one or more of the side walls. In some cases a communication trench connected the emplacement and a nearby bomb-proof coconut-log shelter. The revetments for all emplacements were higher on the inshore sides than on the seaward sides, permitting low-angle antiboat fire on beach approaches. Sandbags secured the fill on top of the revetments.

The guns were mounted on spider-leg pedestal mounts.

Each group of guns was provided with fire-control equipment consisting of a range finder with a 2-yard base and a small (Model 1930) sound locator with four horns. Searchlights (150-cm and 90-cm) were located so as to serve both the antiaircraft and nearby coast-defense guns.

b. For 127-mm Twin-mount Guns

These guns, designed for coastal defense as well as antiaircraft defense, were emplaced in pairs in two locations. Their emplacements were constructed of concrete and were banked from the ground level to the brim of the concrete parapet with coral and sand. The distance between emplacements, center-to-center, was 40 yards.

Spaced at equal intervals around the side of the parapet were 10 ready-boxes, each holding 12 rounds of ammunition. Ammunition stores were kept in four concrete, sand-covered, bomb-proof buildings in the vicinity. Apparently the ammunition was carried from the main storage buildings to the ready-boxes.

A fire-control position for the battery of two twin mounts was located on a sand-covered concrete structure, 30 feet square and elevated 15 feet above the ground. A range and altitude finder and a director were mounted on this. The electrical and communication equipment for the installation was placed inside this structure, which was protected by four 7.7-mm machine guns placed at each of the four top corners. Additional protection for the entire installation was provided by two twin-mount 13-mm machine guns on top of the ammunition storage structures on the flanks.

A 150-cm Model 1933 searchlight was located 100 yards out on each flank of the installation.

10. COAST-DEFENSE GUNS

Japanese weapons on Betio emplaced solely for coast defense included six 80-mm guns, four 140-mm guns, and four 8-inch shielded naval type guns.

a. 80-mm Guns

These guns were found in batteries of three. There were two such batteries, one located in the center of the west beach and the other on the eastern part of the south shore. The latter battery had a secondary mission of covering the south reef antitank barrier with flanking fire.

The emplacements were constructed of logs and sand, and were open at the top except for palm-branch canopies provided for camouflage. They had six sides, with a rear entrance. Double walls, made of logs, formed the six sides. Each log wall was laid horizontally, and was three logs, or 3 feet, high. The space between the double walls was about 3 feet and was filled with sand.

The front side of the emplacements paralleled the beach. Built into the two rear walls were concrete ammunition shelters, each capable of holding three ammunition boxes of 12 rounds.

Japanese standard-type bomb-proof shelters, each capable of holding 12 men, were located to the rear and to one side of each emplacement.

An observation tower, 15 feet high and constructed of logs, stood just behind, and to one side of, the central gun of each battery.

b. 140-mm Guns

Two of these weapons were located on the northwest point, and two on the east point. They were mounted in circular concrete pits which rested on the normal ground level. The floor parapet and ready-boxes of each pit were made of reinforced concrete and banked with sand. The pits, or emplacements, in each battery were about 60 yards apart.

The fire-control arrangement for each pair of guns included an observation tower, 80 feet high, which was erected on a sand-covered, bomb-proof shelter. The latter housed a power distribution board for supplying electricity to the guns and a communication or control center for the battery.

c. 8-inch Shielded Guns

Four of these guns were found on Betio, two on the southwest corner and two on the southeast coast.

The two guns on the southwest corner were mounted in a tandem arrangement in concrete emplacements, which were banked all around with sand and coral.

Ammunition was stored in a heavily constructed bomb-proof shelter, 75 yards from the gun position. A narrow-gauge railroad track led from the storage shelter to an ammunition ready-room which separated the two gun positions. Small hand-drawn cars were used on the track.

The fire-control system for this installation included a plotting room in a lower level of the upper gun emplacement, an observation tower (70 feet high), and the necessary wire and voice-tube communication between these elements and the guns.

The two guns on the southeast coast were located in circular concrete emplacements about 10 feet above the ground. The two emplacements were about 100 yards apart.

Ammunition and powder, and the personnel to handle them, were sheltered in four magazines situated a few yards to the rear of each emplacement. A ready ammunition-handling shack also was situated behind each emplacement.

A fire-control tower, 45 feet high, was situated behind the two guns.

11. TANKS

The Japanese had 14 Model 2595 (1935) light tanks on Betio. They were camouflaged in dug-in revetments, the tops of which were flush with the ground. The camouflage generally consisted of palm leaves.

Nine of the tanks were held around the air-defense command post near the lagoon shore.

12. COMMAND POSTS

The rifle and machine-gun positions, which formed the primary beach defense, were controlled from steel pillbox-command posts, spaced at intervals of about 300 yards around the perimeter of the island.

These pillboxes were prefabricated, six-sided, truncated pyramids with double walls made of steel (see fig. 16). Both the outer and inner plates were 1/4 inch thick, and the space between them was filled with sand. In most cases the exterior of these pillboxes was banked with sand or camouflaged with palm fronds.

Inside, these pillboxes were divided into an upper and a lower compartment. The upper compartment was designed for an observer or commander. It had an observation seat, a voice tube, and a hatch, which opened outwards from near the sides. The voice tubes led down into the lower compartment, which apparently was designed to house two machine guns. This compartment had two large ports, one on the left flank and one on the right, and a small peep sight to the front. The large ports were equipped with bracket supports for machine guns, but apparently no machine guns were used in any of the pillbox-command posts on Betio.

One steel pillbox on the south beach was capped with 12 inches of concrete.

The largest reinforced concrete structure on the island was believed to have been the main headquarters. It housed the terminal facilities of radio and telephone equipment, as well as administrative personnel. The reinforced concrete was very thick. The structure was additionally protected by a thick layer of sand on the roof, on which two 13-mm machine guns were emplaced. Apertures were provided in the walls of the building for the firing of small arms and light automatic weapons, but the primary purpose of the building apparently was to provide shelter for personnel and equipment.

13. SHELTERS

The shelters found on Betio can be broadly divided into two groups: those located in barracks and headquarters areas and those located to serve beach-defense.

Those in the barracks and headquarters areas were designed to protect large groups of personnel during air or surface bombardment. They were constructed of alternate layers of coconut logs and coral sand. Side walls and roofs averaged 5 to 7 feet in thickness. Ventilation shafts were provided, but there were no gun or rifle ports.

Those in the beach-defense areas were designed and used, during bombardment, as standby shelters for personnel who were waiting to man beach positions. These shelters were smaller and not as heavily constructed as those inland. They were located immediately adjacent to defensive positions. Several were constructed of reinforced concrete 12 to 16 inches thick and covered with sand. Others were constructed of logs and sand. They varied in size and design to suit the particular needs of an area. Like the larger structures, the beach-defense shelters were designed for protection, not as prepared defensive positions.