Fortress • 29
Series editors Marcus Cowper and Nikolai Bogdanovic
Introduction
American tactical defense doctrine
Unit defensive principles • Special defensive principles
Building and manning the defenses
Establishing the defense • Conduct of the defense • Defensive firepower
Construction materials
Principles of construction • Camouflage
Types of emplacements
Infantry emplacements • Crew-served weapon emplacements • Trenches and shelters • Obstacles
Theater specific defenses
North Africa • Italy • Northwest Europe • The Pacific • Korea
The test of battle
Normandy • Korea
An assessment of US field fortifications
In terms of strategy, in World War II the United States Army and Marine Corps were offensively oriented. There were few situations in which large-scale defensive operations were necessary, though there were notable exceptions. The beginning of the war found US and Filipino forces on Luzon conducting an army-level defensive operation on Bataan Peninsula. Prolonged defensive corps-level battles for beachhead lodgments were fought on Guadalcanal, Bougainville, and New Britain between 1942 and 1944. In Europe the 1944 defense of Bastogne was America’s most noted large-scale defensive battle. The Korean War began with a defensive battle, the army-level defense of the Pusan Perimeter. After a phase of seesaw offensives and withdrawals, the conflict bogged down into World War I-style positional warfare and to this day the belligerents face each other across the no-man’s land of the Demilitarized Zone.
A Marine 75mm M1A1 pack howitzer is emplaced in a Tinian sugarcane field in an improved crater created by a large-caliber naval shell.
The field fortifications built in the major World War II defensive battles were temporary in nature. Despite the orientation toward the offensive, all units prepared defensive positions when halted for the night or as part of a holding action while other forces advanced. US field fortifications tended to be simple, and as little effort and as few resources as possible were expended on them, even if they were to be occupied for weeks. Even in the prolonged static warfare phase of the Korean War, though much effort went into preparing robust field fortifications, they were still considered temporary.
This 82d Airborne Division 81mm mortar crew of glider troops has dug a near regulation emplacement and camouflaged it reasonably well from ground observation.
The construction of field fortifications and obstacles is prescribed in the US Army’s Engineer Field Manual (FM) 5-15, Field Fortifications. Both the Army and Marine Corps used this manual. The 1940 edition, in use at the beginning of the war, was based on World War I experiences. While it included updated field fortifications for new weapons and more dispersed unit defensive positions, much space was allotted to elaborate trench systems and facilities such as underground kitchens, latrines, and troop shelters. The same applied to the construction of vast in-depth obstacle systems and almost permanent crew-served weapons positions. Hastily dug positions used by infantrymen and their accompanying crew-served weapons were given minimal space. The first three months of America’s first major offensive, Guadalcanal in 1942, proved to be a defensive battle to hold the Henderson Field perimeter. It was realized there that fixed, in-depth defenses were unnecessary against a lightly armed enemy lacking heavy artillery, significant armor, and air superiority. It was here that the Marines learned to copy the Japanese foxhole, dug deep to protect against artillery fire and allow a man to fight standing.
The 1944 edition of FM 5-15 deleted most of the trench warfare and elaborate obstacle information, added simple and quickly dug infantry and crew-served weapons positions, included new types of emplacements to accommodate recently fielded weapons, and exploited the lessons learned in combat. Protection from artillery, tank, and air attack received special emphasis.
The 1949 edition was distributed on the eve of the Korean War, and incorporated the lessons learned in World War II and positions for new types of weapons. It almost appears that the manual was written to support operations in Korea, with information on protective bunkers for crew-served weapons and troops as well as simplified trench systems. This was due to the threat that nuclear weapons now posed on the battleground, but would prove to be well suited for resisting massed artillery barrages experienced later on Korean hills. Additional information was provided on field fortifications in cold weather conditions, some based on German experiences on the Eastern Front.
US forces employed a triangular structure at all echelons from regimental to squad level, and standard practice was for one-third of a given echelon’s subunits to be kept in reserve (“two up, one back”). With relation to frontline strength, the infantry division had three regiments, each with three battalions—a total of nine battalions. Each battalion had three rifle companies for a divisional total of 27 companies. The 27 rifle companies comprised 81 rifle platoons. The two frontline regiments thus had two battalions each in the line for a divisional total of four battalions out of nine. These four battalions each had two rifle companies in the line so that the divisional front was covered by eight out of the 27 companies. Of the division’s 81 rifle platoons only 16 truly manned the frontline. Realistically each company’s support rifle platoon could be considered to be in the frontline as well as it was usually in sight of the two forward platoons and able to provide covering fire—thus meaning there were 24 rifle platoons in the division’s frontline. This second-echelon platoon was referred to as a “support platoon” rather than a reserve. At higher echelons the second-echelon company, battalion, or regiment was designated the “reserve.”
There were of course situations where all three of a given echelon’s subunits were required to man the frontline, especially in quiet sectors or if assigned frontages were too wide. Commanders attempted to avoid the latter, but it was common. In this case, though, some form of reserve was established. For example, a battalion forced to place all three companies in the frontline might place one of the companies’ platoons, reinforced with battalion support elements, in battalion reserve—but the contributing company was deprived of its support.
Care had to be taken when clearing fields of fire in order to ensure they were not conspicuous.
In such circumstances, units other than infantry were often employed as reserves or to man ‘quiet’ sectors. In Europe, Army combat engineer units were frequently employed as infantry, being organized similarly and possessing crew-served weapons. The Marines commonly used engineers, pioneers, amphibian tractor, and even artillery units as infantry, such as on Guadalcanal.
US defensive tactics at the small-unit level were straightforward. The basic fundamentals of defense were essentially the same at each echelon, taking into consideration additional support assets at higher echelons. Troops were not positioned in continuous lines, but in small groups on key terrain. These were referred to as “holding garrisons,” but this term eventually fell from use. Essentially each holding garrison or platoon position was a strongpoint. They defended the terrain features and the unoccupied ground (gaps) between the holding garrison’s battle positions by observation and fire. Gaps may have been protected with obstacles including minefields and indirect artillery fire. They did not defend “lines” (“lines” mean mutually supporting defensive positions roughly abreast, according to the terrain). The main line of resistance (MLR) was an imaginary line along the forward edges of the most advanced defensive positions, essentially the forward defending rifle platoons and squads. When the enemy approached the MLR he was hit first by artillery fire, then cannon and mortar fire, and direct-fire infantry weapons including small arms, machine guns, and antitank (AT) weapons.
The two-echelon defense was essential in order to provide mutual support to the holding garrisons. The fire for each battle position was not only directed to the front, but across the fronts or flanks of other positions adjacent or further forward, plus covered gaps between positions. Battle positions were typically manned by a platoon with reinforcing crew-served weapons from company, battalion, and regiment. If a unit was assigned a wider than normal frontage or the terrain was broken or covered by dense vegetation, some rifle platoons might have been split with two squads manning one position and a single squad manning another. Both were reinforced by machine guns.
Defensive positions were not always established on the best available ground for observation and fire in order to conceal them and to surprise the enemy when he approached. This was a more important consideration than positioning weapons to achieve the longest possible range. Often the aim of a defense was to deny the enemy use of specific terrain, such as a hill. It was not always necessary to physically occupy the feature; depending on the terrain, a hill or bridge could be defended by occupying adjacent terrain from which the feature could be observed and covered by fire. Terrain features providing natural obstacles to tanks and infantry were incorporated into the defense when possible. Swamps, marshes, forests, rivers, streams, gullies, ravines, broken and extremely rocky ground halted or slowed tanks. Obstacles had to be covered by observation and fire to be effective.
Beyond the MLR was the outpost line of resistance usually established by the regiment. During the Korean War this was designated the combat outpost line (COPL). Units manning the COPL were detailed from the reserve battalion and a commander designated. A company or more might be employed for this mission. The outposts were usually placed beyond infantry weapon range and positioned where they could observe the most likely enemy avenues of approach (up to 4,000–6,000 yds forward of the MRL, dependent on terrain). The size of the outposts was determined by terrain, number of avenues of approach, and their mission—either to observe and report, or to engage and delay the enemy. Being beyond infantry weapon range they were usually of platoon strength. Their primary mission was to prevent surprise attacks and hamper enemy patrols. The outposts were concealed and only simple positions constructed. Few if any crew-served weapons accompanied the outposts, as they had to be able to withdraw quickly. Mortars and cannons supported them though. Once withdrawn they would re-occupy their reserve positions. Because of the proximity of the enemy such outposts were seldom established. If the regiment did not establish outposts, battalions and companies would establish smaller outposts on the outpost line of observation, of squad strength. These might be up to 400 yds forward and within range of infantry weapons on the MLR. Smaller observation posts and listening posts (OP/LP), comprising 3–5 men, could be established. The OPs were employed further out during the day to observe avenues of approach and at night moved closer to the MLR to serve as LPs. OP/LPs also covered gaps between positions and screened exposed flanks.
The regimental, battalion, and company (support platoon) reserve could be employed in a number of ways. (1) It could reinforce a forward position when it was determined the enemy main attack would fall there. (2) It could be employed as a counterattack force if a position was overrun. (3) It could relieve or reinforce a position under heavy attack. (4) It could remain in a second-line position to block an enemy penetration of the MLR or maneuver to another blocking position. (5) It could move to a position to secure an exposed flank. The reserve unit (support platoon) may have prepared second-line defensive positions on defendable terrain, but then the unit would be positioned in a defiladed area and simple positions dug for protection from artillery. Time permitting, the reserve company may have prepared two or more positions for each platoon in order to block different routes through the reserve area and protect the flanks. The reserve commander would reconnoiter routes to the different second-line positions and counterattack routes toward the forward positions.
High ground was always desirable for its observation advantages, extended fields of fire, and because it is harder to fight uphill. A rule of thumb was to position defenses on the “military crest,” a line along the slope of a hill from which maximum observation up and down the slope can be obtained. However, if at all possible the positions were placed where they could observe and fire on the high ground’s base. Positions were not to be established on or just below the topographic crest as they were silhouetted against the sky. In the desert even an elevation of a few feet would be an advantage. Natural terrain obstacles were tied into the defense whenever possible. The routes and directions of possible enemy attacks were determined and infantry and supporting weapons were designated to cover those approaches. The goal was to destroy or disrupt the attackers by concentrating all available fire before the enemy reached the MLR. Effective employment of the different weapons organic to a regiment was an art in itself as each had capabilities and limitations to be considered.
Camouflage and all-round local security was continuous during the development of defensives. Camouflage had to deny or limit the ability of the enemy to detect positions from the ground and air. Platoon positions were to be prepared for all-round defense, although limited time did not always allow this. A unit did not attempt to man a complete 360-degree perimeter. Riflemen and crew-served weapons were mostly oriented to the front with some on the flanks to cover gaps between positions. Time permitting, supplementary positions were prepared to fully defend the flank and rear of the position. Alternate emplacements were sometimes prepared for crew-served weapons. These covered the same sector of fire as the primary position if it became untenable. Supplementary positions covered other sectors, to the flank or rear for example.
Organization of the defense was accomplished by a number of control measures. The MLR and outpost lines have already been discussed. Boundaries were established between units at all echelons. This restricted movement and fire into adjacent unit sectors. A unit could send patrols or maneuver into or even fire into an adjacent unit’s sector, but this had to be coordinated. Behind the two forward battalions a regimental reserve line (RRL) was designated. Behind the RRL was the reserve battalion and various regimental facilities to include the regimental command post (CP), aid station, ammunition supply point (ASP), service company, and CPs and rear elements of the AT and cannon companies. Contact points were designated on maps where unit boundaries crossed the MLR and RRL. It was here that commanders or their representatives would physically meet, terrain and situation permitting, to coordinate fire and observation. CPs, ASPs, and aid stations were also established in company and battalion areas. A rear regimental boundary delineated the regiment’s rear area. Behind this was the divisional rear area, which included the reserve regiment, division artillery, attached and reinforcing artillery, and service support units and facilities.
These 24th Infantry Division positions in Korea are dug in along the crest of knife-edged ridges rather than lower down the slope on the military crest, as was doctrine. Such ridge-top positions were difficult to hit with artillery and mortars.
The frontage of a given unit was dependent on terrain, visibility, available strength, supporting weapons, and just as importantly, the kind of force the enemy could commit. Subunit sectors on a given piece of terrain might not have been of the same width. A regiment, for example, may have assigned one battalion a narrow sector on a favorable enemy avenue of approach to allow it to concentrate its firepower, while the other battalion may have had a broader front as it offered a less favorable approach. In such cases units assigned the broader frontage had to be in a position in which the possible loss of ground would not threaten the defense as a whole. Units could be assigned a wider frontage when the enemy was forced to advance over exposed ground. Table 1