A well dug-in secondary line was located four to eight miles to the rear of the Chinese MLR. Intelligence indicated that an attack to infiltrate CCF defenses would “require the penetration of a fortified area to a minimum depth of 10 miles.”[766] Some Korean War analysts maintained that behind their front line the Chinese had entrenched the ridges to an average depth of 14 miles and that the enemy “could have fallen back upon successive prepared positions for all that distance.”[767] Although the trench warfare period of the Korean War was often likened to World War I, the Chinese defensive works were estimated to have “ten times the depth of any belt of entrenchments in World War I.”[768] Some areas had even been engineered for defense against nuclear attack. Caves, tunnels, and particularly reverse slope positions also showed CCF skill in the selection and organization of terrain features.
[766] PacFlt EvalRpt No. 5, p. 8-29.
[767] Marshall, Pork Chop Hill, p. 24.
[768] Ibid., p. 116.
Both the nature of the ground fighting and weather in Korea quickly indicated that our bunker construction needed to be improved. Siting them lower into the ground, so that the outline of the bunkers would not make them such ready targets, and reinforcing them to withstand a 105mm direct hit were steps in this direction. Use of sandbags (of which there was a continuing shortage) for both bunkers and trenches proved to be almost as much a problem as a solution. Bunkers above ground shored up with sandbags frequently collapsed in times of heavy rains or Korean spring thawing conditions.
Outpost warfare also proved that the average bunker often became a deathtrap when used defensively. This was due to the enemy proclivity for sealing entrances with their satchel charges, as occurred in the Vegas Cities battle. It became evident that large living-fighting bunkers could easily turn into traps in which many men could become casualties simultaneously, and from which few could fight. Despite their exposed nature, fighting holes were often safer. Some Korean combat officers were of the opinion that rather than our six- to eight-man bunkers, smaller two-man fighting units would be obviously faster to build, more effective, and safer since they would present a smaller target.
A 1st Marine Division training bulletin issued near the end of the war stated categorically:
As a rule no bunker or cave should be large enough to accommodate more than four men. If the cave is bombardment proof, there is another greater danger that the men will fail to man their fighting positions quickly enough after the enemy fire lifts or ceases.[769]
[769] Lessons Learned, 5-53, p. 11.
UNC reconnaissance and security activities also showed need for improvement. Night raids, patrol operations, and ambushes were conducted continuously to maintain contact with the enemy, keep him off balance, and obtain intelligence. This type of mobile, small-unit action repeatedly indicated an urgent need for more basic training in night combat operations at the squad and platoon level. The frequent breakdown of communications in night fighting, whether it involved a small patrol or besieged outpost, was particularly critical. Some regimental commanders noted the failure to employ properly organic small arms in combat action during darkness before requesting heavier supporting fires. It was felt that the practice of calling for mortar or artillery fire to the exclusion of using small arms was a dangerous practice which was being overused and that “even in the defense the spirit of the offensive must be maintained.”[770] Meticulous planning was vital for effective fire plans, alternate avenues of approach, and evacuation. Detailed rehearsals of raids were essential.