The Pacific War Online Encyclopedia

Terrain

Photograph of rugged terrain at Shuri, Okinawa

U.S. Marine Corps. Via ibiblio.org

Military operations have always been strongly affected by the terrain of the battlefield, which affects movement and provides cover.

Movement. Flat, firm ground favors rapid movement by men and vehicles. However, in the presence of even modest rainfall, heavy military traffic could quickly pound seemingly substantial terrain into nearly impassable mud. Thus, engineers were required to build roads that could take heavy traffic. Where gravel and construction equipment was available, macadam roads could be constructed. At other locations, corduroy roads could be built from local timber, though this was an unpleasant surface for vehicular movement. United States engineers often made use of Marston mat to rapidly construct roads under less than ideal conditions.

Airfields required large areas of flat, firm terrain and were built in essentially the same way as roads. Suitable terrain for airfields was precious enough that entire campaigns were fought for it. In New Guinea, the Allies discovered that ancient river beds, whose accumulated gravel provided a suitable base for airfield construction, were often marked by fields of kunai grass.

In the equatorial jungles of the South Pacific, flat ground often lacked sufficient drainage to shed the torrential rainfall, which turned the terrain into swamps. Swamps were impassable to any kind of vehicle and all but impassable to men on foot. Relatively solid paths through swamps became killing grounds, as an advance was channeled along the paths and subject to heavy fire from defenders who were acquainted with the terrain.

Marine command post taking cover in gully. Via ibiblio.org

Cover. Vegetation provided much of the cover in the South Pacific and elsewhere, but the underlying terrain was also critical. Possession of the high ground favored both attackers and defenders, as troops could move behind a crest line unobserved. In addition, some kinds of terrain were more favorable to entrenchment than others. Entrenchment was all but impossible in either waterlogged terrain, where the water table was struck just below the surface, or in rocky terrain such as the fossil coral reefs found at Peleliu and other locations. On the other hand, fossil coral with its many caves was ideal for construction of prepared positions using explosives and heavy equipment.

Effects on combat. In general, rugged terrain favors the defense. Even under the best of conditions, an attacker was reckoned to need a fire superiority of three to one to overcome the natural advantages of the defender, whose men could take maximum advantage of any available cover and whose heavy weapons could be predeployed with maximum fields of fire. Thus an attacker needed to be able to concentrate his forces faster than the defender. An attack was a race to break through the defender's lines before the defender could bring up reserves to reduce the attacker's fire superiority to an ineffective level.

Rough terrain usually favors the defender in this race, who is able to bring up forces along routes that are not under enemy observation, while the attacker must frequently break cover to sustain his attack. However, skilled commanders were sometimes able to take advantage of terrain while attacking, particularly against inept defenders, as was the case for the Japanese in Malaya. The poorly-trained Allied troops were very reluctant to move off the established highways into the jungle, which allowed the Japanese to infiltrate through cover and surround Allied units before the Allies knew they were there.

Combat environments in the Pacific Theater

References

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