The Pacific War Online Encyclopedia
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U.S. Army. Via ibiblio.org.
Fortifications are engineered
defenses designed to reduce the number of troops required to defend a
position. Strategically, they serve to release troops for other
operations. They cannot win a battle by themselves, since they are
immobile. They can always be reduced, in time, by a sufficiently large
enemy force, unless a relieving force throws the enemy back.
Fortifications are divided into fixed fortifications and field works.
U.S. Army. Via ibiblio.org
Fixed fortifications are hardened permanent structures whose construction often takes considerable time and resources. For this reason, they are often constructed in peacetime as permanent components of the national defense infrastructure. Fixed fortifications include large-caliber coastal defense guns emplaced in concrete and steel casemates, such as those that defended Singapore, Manila Bay, and other major ports; systems of concrete pillboxes forming a defensive line along a strategic frontier, such as the Gin Drinker's Line at Hong Kong; and other similar fortifications.
U.S. Army. Via ibiblio.org
Field works are improvised fortifications that are
constructed during hostilities by armies in the field. Field works are
further broken down into hasty field works and deliberate field works.
Hasty field works include fox holes and open camouflaged positions
protected by barbed wire, which are constructed on a time scale of
hours. Deliberate field works include systems of trenches and covered
positions protected by minefields, which are constructed on time scales
of days to weeks. The construction of hasty field works is colloquially
known as "digging in," and the ability to rapidly dig in after a
successful attack was key to repelling subsequent counterattacks. The U.S. Army estimated that it took 90
minutes for a soldier to excavate and camouflage an individual
rifleman's foxhole, up to 8 man-hours to fully prepare a machine gun
pit, and up to 28 man-hours to fully prepare an antitank gun position.
Infantry were provided with entrenching tools as part of their standard kit. The standard U.S. entrenching tool was designed so the blade could fold up for easier carrying, be fully extended when used as a shovel, or be partially extended to be used as a pick. Tool packs that included crowbars, saws, axes, wire cutters, picks, and full-sized shovels were issued as needed to infantry formations, and barbed wire, mines, and other matériel for fortification were stockpiled by engineers. The Japanese and British also issued entrenching tools to their infantrymen, and the British version had a bayonet lug to allow the entrenching tool to be converted into a mine prod.
A major hindrance to fortification in many parts of the Pacific was the high water table, which meant that deep fortifications would gradually fill with water. Even where the water table was relatively low, heavy rainfall could flood hasty entrenchments. Deliberate entrenchments could be constructed with drainage channels where the water table was low enough, but this took additional effort. Damp soil also flows under its own weight, which meant that excavated positions could collapse on their occupants unless the excavation was stabilized with sandbags, cut sod, lumber, burlap and chicken wire, or expanded metal or corrugated iron. The Marines at Iwo Jima found that the volcanic ash covering the island had almost no ability to support its own weight even when dry, and the inability of the troops to quickly entrench increased their vulnerability to heavy Japanese fire.
Another hindrance to fortification was rocky ground. A number of islands, including Peleliu and Los Negros, were characterized by terrain consisting of a thin layer of soil over almost solid coral.
Fortifications include concealed and protected positions,
cleared fields of fire, communications, and obstacles.
Concealed Positions. Concealed positions make use of terrain and natural vegetation, supplemented at times with artificial camouflage, in order to be difficult for the enemy to spot. This allows troops in the position to surprise an unwary attacker and slow an attack by forcing the attacker to scout the area. Well-trained troops were careful to remove natural sod and debris from both the site to be excavated and the location where the spoil (excavated material) would be placed; the sod and debris could then be used to conceal both position and spoil. Topsoil was kept separate from the (usually lighter) subsoil and used to conceal the subsoil spoil. Concealed positions are largely a feature of hasty field works, since fortifications that are in place for a long time are likely to be well scouted by any potential attacker. However, even concealed positions known to the enemy are more difficult to hit with long range fire, since the effects of the fire are difficult to observe.
The Japanese were generally skilled at concealing positions
and in the use of camouflage. Where time permitted, vegetation was
deliberately planted on top of positions and camouflage was carefully
blended with the natural surroundings. On palm-covered atolls, the
Japanese would place palm fronds around positions and these would be
indistinguishable from palm fronds knocked down by the Allied
prelanding bombardment. Japanese troops were issued camouflage nets
with green, brown, and tan strips woven through the nets. However,
dazzle patterns for large structures, such as concrete pillboxes,
bunkers, hangars, warehouses, and so on, were less skillfully done than
Protected Positions. Protected positions offer physical protection from enemy fire. They range from a hastily dug foxhole to massive concrete and steel casemates requiring months or years for construction. Hasty field works usually offer little overhead protection, which is thus a distinctive feature of deliberate field works and fixed fortifications. However, the U.S. Army assumed that ordinary trenches provided adequate protection from small arms, and from anything but a direct hit from shells of less than 4" (105mm) caliber or bombs of under 50 pounds (23 kg). Protection could be enhanced by placing the position in defilade, that is, taking advantage of terrain features to protect the position from direct fire.
Overhead protection for deliberate field works ideally consisted of a surface camouflage cover of earth and natural vegetation, underneath which was a burster layer designed to activate fuses on bombs and shells. American forces typically constructed the burster layer from prefabricated concrete bursters, reinforced concrete beams, concrete or brick rubble, or poured concrete. Underneath was a cushion layer, ideally a layer of gravel on a layer of tamped earth, and beneath the cushion layer was a distributing layer of wired logs, concrete beams, or iron I-beams to transmit forces to the sides of the fortified position. There might be multiple alternating cushion layers and distributing layers in the strongest fortifications. The U.S. Army estimated that a 5" (13 cm) reinforced concrete bursting layer was adequate to protect against a direct hit from a 75mm shell, while 16" (41 cm) was required against 155mm shells and 24" (61 cm) was required against a 100 lb (45 kg) general purpose bomb. In general, reinforced concrete was twice as effective as wired logs, over three times as effective as coarse gravel, and seven times as effective as tamped earth.
U.S. Army. Via ibiblio.org
Antiaircraft positions were very difficult to conceal, since they generally needed to be sited in open areas with clear fields of fire in all directions. They were also difficult to protect, because they could be given little or no overhead protection. However, the Japanese developed a "double donut" emplacement consisting of two concentric parapets around the gun, with the openings offset, and perhaps some gesture of concealment in the form of camouflage nets or palm fronds.
U.S. Park Service
A unique feature of Japanese fortifications on Betio at Tarawa was the presence of prefabricated
steel pillboxes left over from the Russo-Japanese War. These were
hexagonal in plan, measured about 9' (2.7m) across the bottom and
tapered to the top, and were just over 8' (2.4m) high. They had double
walls 0.25" (6mm) thick with the layers spaced a foot (30 cm) apart and
the space filled with sand. They were used as company command posts but also housed two heavy machine guns.
Fields of Fire. Concealment or protection are of little use if a position does not command a useful field of fire.
Clearing a field of fire may mean hastily removing some vegetation
from immediately in front of a machine gun pit or completely clearing the area
within several hundred yards of a fixed fortification. There is a
obvious trade off between clearing fields of fire and maintaining
concealment. Good fields of fire are also a function of terrain, and
positions on high ground have been favored for their superior fields of
fire since the introduction of arrows and spears to warfare. However, a
position that can see the entire battlefield is a position that can be
fired upon from the entire battlefield, and positions on high
ground were often placed on the reverse slope, sacrificing some of
their field of fire for the protection of being in defilade.
The U.S. Army estimated that it took 90 man-minutes per 100 square
yards (84 square meters) to hastily clear a field of fire, and
3.5 man-hours per 100 square yards for more deliberate clearance of
fields of fire.
The Japanese were particularly skilled at clearing fields of fire. Low
branches and vegetation were carefully cleared from selected firing
lanes while higher branches were left in place, so that the approaching
enemy were unaware they had walked into a firing lane.
Top view of a stepped port
Just as there is a tradeoff between commanding a good field of fire and being exposed to fire, there is a tradeoff between firing ports that give large firing arcs and firing ports that provide adequate cover. Concrete ports were often built with a wide outer opening and smaller inner opening to maximize protection while maintaining good firing arcs, but such ports had to be constructed in steps so that they did not funnel projectiles into the inner opening.
U.S. Army. Via ibiblio.org
Communications. Secure communications include communications trenches or tunnels between positions and protected telephone lines. The Japanese sometimes constructed tunnels from oil drums with their ends removed and laid end to end. Telephone lines takes significant time and effort to lay down and so are a feature only of fixed and deliberate field fortifications. Telephones had a significant advantage over radio in their reliability and security, though artillery could sometimes cut telephone cables and enemy infiltrators could cut or tap into telephone lines. In fixed positions, or where there was ample time for constructing field works, telephone lines could be buried a foot (30 cm) underground, which gave them somewhat greater resistance to artillery fire and concealed them from infiltrators.
U.S. Army. Via ibiblio.org
Obstacles. Obstacles hinder enemy movement. They include barbed wire, antitank ditches, iron tetrahedra, "dragon's teeth" (large concrete roadblocks), and the deliberate flooding of low-lying areas. More primitive improvised obstacles include the punji stick, a sharpened bamboo or wooden stick placed upright in the ground where an unwary enemy soldier might step on it and suffer an ugly wound to his food or leg. Punji sticks were first encountered by the British in Burma in the 19th century, but were used by American and Filipino troops in Bataan, who lacked mines or barbed wire with which to construct more conventional obstacles.
A variation on the punji stick, which was taught to the Kachin guerrillas
of northern Burma, was the cartridge trap. This was a simple improvised
booby trap consisting of a rifle cartridge set atop a buried spike so
that the primer of the cartridge rested on the spike and the top of the
bullet was just above ground level. An unwary soldier stepping on the
cartridge would force the primer down on the spike, firing the
cartridge. Although the force of an unconfined cartridge was much less
than that of a cartridge fired in a gun barrel, it was often still
sufficient for the bullet to penetrate the soldier's foot and render him
hors de combat.
U.S. Army. Via ibiblio.org
Barbed wire could be laid in a number of patterns,
depending on how much barbed wire was available and how much time was
available for arranging it. When supplies were short, nothing more
could be done than to string a few strands along low posts where they
were difficult to spot. When wire was plentiful, single or double apron
barbed wire barriers could be constructed. Concertina wire was barbed
wire shaped into coils so that it could be laid out as large coils
resting on the ground.
U.S. Marine Corps. Via ibiblio.org
booby traps are also considered obstacles, even though they are designed
to inflict casualties,
because they instill caution in advancing troops, who must either take
time to clear the mines and booby traps or suffer the consequences for morale
of advancing into a known minefield. Antipersonnel mines were designed
to detonate when activated by the weight of a single soldier, while
antitank mines carried a larger explosive
charge and were activated only by the weight of a heavy vehicle. (The
latter should not be confused with antitank "mines", actually demolition
charges, attached to a tank by an intrepid infantryman using magnets or
The principal Japanese antipersonnel mine was the Type 93 mine, which was detonated by a force of 20 lbs to 250 lbs (9kg
to 110kg) anywhere on its upper surface. It contained about 2 lbs (0.9
kg) of explosive, but at the maximum pressure setting and with
additional explosives emplaced under the mine, it made a serviceable
antitank mine. The American counterpart was the M3 antipersonnel mine,
which could be triggered either by 20 to 40 lbs (9 to 18 kg) pressure
or a tripwire and which contained 0.9lbs (0.4 kg) TNT in a heavy cast
iron case. The fragments were expected to be lethal within a 10 yard
(9m) radius when emplaced on the ground, or up to 100 yards (90 m) when
detonated several feet above the ground (as when deployed with a
tripwire.) The Americans also developed their own version of the German
S-mine, the M2A3, which when triggered fired a large grenade a few feet
into the air, where it exploded with maximum effect.
The American antitank mine was the M1A1, which contained a 5.8 lb (2.6 kg) charge of TNT detonated by a pressure of 250 to 500 lbs (110 to 220 kg). The Japanese antitank mine was dubbed the "yardstick mine" by American troops, as it was a yard (36" or 91cm) long and 3.35" by 1.8" (8.5cm by 4.5cm) square. It contained eight blocks of picric acid explosive totalling 6 lbs or 2.7 kg and was fused to explode under a weight of 336 lbs (152 kg). Both Japanese and American antitank mines had enough explosive power to break the track on a tank, but the Japanese often planted enough additional explosives under the yardstick mine to completely destroy a tank.
In general, the Japanese did not make effective use of mines until after the invasion of Saipan in June 1944. Mines were poorly sited and inadequately camouflaged. American
troops encountered extensive minefields in the Philippines, Iwo Jima,
and Okinawa, mostly behind likely landing beaches.
In the early months of the war, the Japanese benefited from the naval disarmament treaties, which had greatly restricted the fortification of Allied territories in the western Pacific. The only significant fortresses the Japanese had to reduce were Corregidor and its outlying batteries (Fort Hughes and Fort Drum) and Singapore. The Americans had begun fortifying Wake Island, but the fixed fortifications were still very far from complete when war broke out. The fortifications at Singapore were designed with attack from the sea in mind, and were almost useless against the land campaign down Malaya that actually developed. Corregidor did in fact prove costly for the Japanese to reduce, but with no possibility of relief, and with the Japanese in complete control of air and sea, the fortress could not hold out forever. Wake successfully repelled the first Japanese amphibious assault against it, in spite of its incomplete fortifications, but succumbed to a second assault with massive air and sea support after Pye decided to call off a relief operation.
Elsewhere the Allies were dependent on field works and obstacles. Barbed wire was in short supply, as were mines, and the British in Malaya showed an astonishing reluctance to entrench, with commanders claiming that this was bad for morale. The Americans in the Philippines build extensive field works in the Bataan Peninsula that proved costly for the Japanese to assault; but, with inadequate supplies and no hope of relief, the field works could only delay the inevitable.
At Guadalcanal the Marines rapidly entrenched themselves and set out barbed wire in front of their lines. The barbed wire was typically strung with noisemakers, such as tin cans containing a few pebbles, that would alert the Marines to any infiltration of their front. These defenses proved effective at the Battle of the Tenaru River and were important also at the Battle of Bloody Ridge.
U.S. Marine Corps.
By then the Allies had gone over to the offensive, and were beginning to discover just how skillful the Japanese were at constructing fortifications. The Japanese Army's doctrinal emphasis on the offense meant that Japanese officer had little training in defensive tactics, such as construction of fortifications, but the Japanese learned quickly. Japanese infantry were particularly skilled at the use of the concealed one-man fighting holes which they called "octopus pots" from their resemblance to cookware used to boil cuttlefish. The Allies dubbed these "spider holes" from their fancied resemblance to the den of a trapdoor spider.
Australian and American units closing in on Buna discovered that the Japanese engineers in the beachhead had constructed formidable field works from coconut logs. The spongy wood of the coconut logs absorbed shock very effectively, and the coconut log bunkers quickly were overgrown by jungle vegetation that provided excellent concealment. The bunkers were roofed with additional coconut logs, sometimes reinforced with steel beams, making them impervious to infantry mortars. Furthermore, the bunkers were so positioned that they had interlocking fields of fire; that is, each bunker was within the field of fire of neighboring bunkers, so that troops closely assaulting a bunker could be fired on by its neighbors.
Larger bunkers were often subdivided by internal
partitions, so that if one position was knocked out by flame throwers,
shells, or explosives, the other positions remained active. Japanese
troops sometimes dug grenade wells in their bunkers or foxholes, which
were deep holes into which a Japanese soldier might attempt to move a
live grenade before it exploded. The explosion would be directed
straight up, and the soldiers in the position could gain considerable
protection by lying flat on the ground away from the well. Grenade
wells also served as drainage sumps when constructed in loose soil well
above the water table. Bunkers were also protected against grenades
with small ditches in front of ports to catch rolling grenades and
blast walls behind entrances to catch grenades thrown into an entrance.
The Japanese standardized the construction of fixed and deliberate works, based on the kind of firepower the bunker was expected to withstand (Rottman 2003):
||Offers protection from
||Rock and coral
||Rock and brick
||Sand and soil
||1 ton bomb or 16" (406mm) shell
||500 lb (227 kg) bomb or 8" (203mm) shell
||250 lb (113 kg) bomb or 6" (152mm) shell
||100 lb (45 kg) bomb or 3" (76mm) shell
||25 lb (11 kg) bomb or large fragments
||1' (30 cm)
||13.2mm bullets or small fragments
||2.5" to 4" (6.4cm to 10cm)
||9" (23 cm)
||9" (23 cm)
However, while the basic layout and construction of
fortifications was dictated by Army field manuals, the Japanese showed
considerable flexibility, particularly in construction. Japanese logistics being as
stretched as they were, Japanese engineers often had to make do with
U.S. Army. Via ibiblio.org
U.S. Army. Via ibiblio.org
The Japanese made particularly heavy use of antitank ditches and other antitank obstacles and antitank mines to compensate for their inferior antitank weaponry. The Japanese were frequently short of purpose-built antitank mines and improvised, using aircraft bombs or large artillery shells, which were either electrically fired remotely or detonated by a soldier in a spider hole with the bomb. The latter was encountered in Burma (Allen 1984):
They had been over this ground the day before, and [Colonel Alan Wakefield] now noticed that it was covered in pock marks in regular lines. He told the tanks to stop and cover him with their guns, then went forward and began to shoot each human mine in the head. Every now and then he would pause to reload, then move on to the next series of pock marks. None of the Japanese detonated their bombs— Wakefield was not a tank, and they had been told to do it only for tanks — and he remained miraculously unscathed by fire from the Japanese in the railway station ... The ground was clear, and the tanks rolled on.
Where the terrain permitted, Japanese fortifications tended to be sited on reverse slopes or other locations where they were not exposed to fire except from relatively short range. This was meant to minimize the firepower advantage of the Allies, though at the cost of restricted fields of fire.
One weakness of Japanese fortifications was the elaborate
systems of communications trenches promulgated in field manuals. These
reflected the lack of experience in trench warfare of the modern
Japanese army, and were relatively easily overrun.
Infantry attempting to assault a bunker had little choice but to attempt to get close enough to throw grenades through the firing slits. Ideally, the bunker's occupants, as well as those of neighbor bunkers, should have been pinned down first by supporting fire from automatic weapons; but the excellent cover provided by the bunkers made fire superiority extremely difficult to achieve. Most soldiers attempted to put a grenade through a bunker's firing slits only once.
The bunkers at Buna were eventually reduced with the aid of artillery and tanks. The Australians contrived to bring in 25-pounder artillery pieces by barge, and the heavy shells of these weapons were able to tear apart the Japanese bunkers, or at least stun their occupants enough to allow a successful infantry attack. The Japanese lacked antitank weapons, and so tanks were able to approach bunkers with impunity, providing cover for friendly infantry and direct fire support with their guns, which were able to shoot into the firing slits of the bunkers.
Similar bunkers were encountered at the Gifu position on Guadalcanal, where the Japanese had built a line of about 45 bunkers. Each bunker was dug into the ground so that it projected only three feet (one meter) above the surface, with walls two logs thick and roofs three logs thick. Earth was thrown up around the walls to provide concealment and further protection. There was room in each for about one or two machine guns and their crews plus two or three riflemen. Nothing short of a direct hit from a 105mm gun was capable of destroying such a bunker. It is a tribute to the hardiness of the Japanese that their hungry, ragged engineers were able to construct such bunkers in such numbers in the depths of the jungle.
U.S. Army. Via militaryphotos.net
Later in the war, the Allies made extensive use of flamethrowers to reduce bunkers and pillboxes. The flaming napalm could be splashed through firing slits to cause direct casualties or to force the defenders to retreat for lack of oxygen (consumed by the burning fuel.) However, the flamethrower operator was highly vulnerable to counter fire because of the bulky fuel and propellant tanks he was carrying.
During the New Guinea campaign, in which the Americans
encountered large numbers of pillboxes, standard tactics were worked
out for reducing a pillbox. First, thorough reconnaissance
was performed to determine the enemy positions, strength, and
intentions. Second, a reconnaissance in force was carried out by a
reinforced platoon to further uncover the enemy positions. Third,
an artillery bombardment was carried out to strip vegetation and other
cover, damage enemy fortifications, and kill, wound, and demoralize
enemy forces. Fourth, 81mm mortars would provide close support to the
infantry. For this purpose, 60mm mortars were found ineffective, but
the 81mm shell was heavy enough to penetrate the jungle canopy and
sometimes the tops of the pillboxes themselves. Finally, the infantry
would carry out a single or double envelopment and attack with small
arms, grenades, and flame throwers.
As the Allies advanced into areas long occupied by the Japanese,
such as the Gilberts and Marshalls, they began to
encounter concrete pillboxes.
These were constructed by the Japanese
wherever there was time and adequate materials. The ideal pillbox had
thick reinforced concrete walls and roofs, sometimes with added layers
of earth or steel rails. They were provided with narrow firing slits
that allowed the occupants of the bunker to fire on approaching troops
while exposing only a few square inches of their own body area.
Concrete is extremely strong in compression. The steel rods
incorporated in reinforced concrete provide strength in tension as
well. As a result, reinforced concrete resists both the strong shock
(compression) and tearing forces (tension) produced by high explosives.
Concrete is impervious to small arms
fire, and only a direct hit from a large-caliber shell was likely to do
significant damage. These bunkers could be reduced by accurate naval gunfire, especially
plunging fire, but all too often the infantry had to reduce surviving
bunkers with tanks, high explosive charges, and flamethrowers.
The Japanese became adept at constructing strong points out of the limestone caves that are so common in the islands of the Pacific. These caves had walls and ceilings made of the natural equivalent of concrete, but far thicker than those of any man made bunker. The caves could be enlarged and interconnected, and the Japanese often put steel doors at the entrances of the larger caves. Japanese artillery could be wheeled out, fire a few rounds, then retreat behind the steel doors, which were impervious to counter battery fire. Such fortifications proved extremely costly to reduce at Peleliu and Okinawa.
The chief disadvantage of concrete or solid rock, aside from the
expense, was its tendency to spall when hit by a large-caliber shell or
subjected to a strong explosive shock. Fragments of rock or concrete
would tear off the inside surface of the structure and could inflict
significant damage. This could be prevented by lining such
fortifications with sandbags, which absorbed spall fragments.
A final twist was provided by the soft volcanic terrain of Iwo Jima. Here the volcanic tuff was easily tunneled yet resistant to shock. Furthermore, the volcanic ash covering the island was discovered to make superb concrete when mixed with Portland cement. The Japanese were estimated to have constructed at least 300 separate fixed fortifications throughout the island, ranging from spider holes large enough for just a single soldier to multistory underground headquarters. Perhaps half were knocked out by naval bombardment, but the remainder were enough to make Iwo Jima the bloodiest battle of the Pacific War in proportion to the number of troops involved.
In addition, most of the thirty tanks of 26 Tank Regiment were buried up to their turrets in the soft volcanic ash, becoming steel pillboxes. This made them more difficult targets but sacrificed their mobility. Since the battle was characterized by positional warfare with little room for maneuver, this doubtless seemed like a reasonable trade off to the Japanese.
It has been alleged that the Allied commanders briefly
considered the use of mustard gas
to flush the Japanese out of their defenses, but the suggestion was
dropped. Mustard gas might well have proven highly effective, since it
is heavier than air and readily sinks into confined underground spaces.
The result would have been about as many Japanese dead (i.e., almost
all of them) but possibly many fewer Allied casualties. However, the
political and moral cost and the
risk of retaliation in kind was
FM 5-15 (1940-10-1, 1944-2-14; accessed 2013-5-10)
Handbook on Japanese Military Forces (1944-9-15; accessed 2013-5-10)
"Japanese Tank and Antitank Warfare" (1945-8-1; accessed 2013-7-24)
The Pacific War Online Encyclopedia © 2007-2008, 2010-2011, 2013-2014 by Kent G. Budge. Index
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