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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 include fixed fortifications, which are hardened structures providing cover for defending troops; field works, which are rapidly improvised forms of cover such as trenches; and obstacles, such as barbed wire, minefields, or antitank ditches, which hinder enemy movement.
In the early months of the war, the Japanese benefitted 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 actuallly 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.
By then the Allies had gone over to the offensive, and were
beginning to discover just how skillful the Japanese were at
constructing fortifications. 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 would be fired on by its
neighbors.
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 attemped 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 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.
Later in the war, the Allies would make 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.) On the other hand, the flamethrower operator was highly vulnerable to counterfire because of the bulky fuel and propellant tanks he was carrying.
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 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.
Japanese logistics being as stretched as they were, Japanese engineers often had to make do with local materials. The Japanese became adept at constructing strongpoints 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 manmade 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, where they were impervious to counterbattery fire. Such fortifications proved extremely costly to reduce at Peleliu and Okinawa.
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. 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 (e.g., almost all of them) but possibly many fewer Allied casualties. However, the political and moral cost and the risk of retaliation in kind was considered unacceptable.
References
The Pacific War Online Encyclopedia (c) 2007-2008 by Kent G. Budge. Index
