In June, massive North Korean forces invaded South Korea. By July, the North Koreans were forcing Republic of Korea (ROK) and American units to the southern tip of South Korea. The 3d Engineer Battalion, 24th Infantry Division arrived in country during the first week of July. Within days, Company B, supporting the 12st Regimental Combat Team [RCT], prepared bridges for demolition, constructed roadblocks, and blew craters in roads. Company D, supporting the 34th RCT took on similar tasks. Working night and day, often under mortar, artillery, small-arms fire, the engineers laid antipersonnel minefields and placed numerous demolitions. Teams were posted to prevent premature blowing of bridges and craters. At one point, Company B had to replace a railroad bridge destroyed by North Korean aircraft. The tracks were repaired in 5 hours, and trains resumed their movement along the line. In a 17-day period, the 3d Engineers made 14 reconnaissances, maintained 1 airstrip and completed another, repaired 25 roads, built 1 bridge, destroyed 56 highway and 11 railroad bridges, blew 19 road craters, laid 1 antitank and 1 antipersonnel minefield, and blew 6 tunnels. In the coming weeks, the battalion would be deployed as infantry, spending 17 days in the line or in active reserve. In the period from July 23 to August 25, the battalion blew 24 road craters; destroyed 1 railroad and 12 highway bridges; installed 4 antipersonnel and 3 antitank minefields; constructed 5 airstrips, 3 fords, and 3 bridges; and repaired 68 miles of road.

Delays

Engineers can expect to play a major role in the maneuver commander's overall plan for a delay. Units conduct delays when their strength is insufficient to attack or defend or when they want to maneuver the enemy into an area for a subsequent counterattack. Delays¯

Gain time for units to reestablish the defense.
Cover a defending or withdrawing force.
Protect a friendly unit's flank.
Participate in an economy-of-force effort.
Slow or break up the enemy's momentum or do not allow it to begin. The commander's intent must specify the duration of the delay, the terrain requirements, and what operations to expect. Engineers plan the assets needed to support the delay. They also consider the enemy situation. The enemy's capabilities have a direct impact on planning and task-organizing for the delay. Engineers should examine the enemy's vulnerabilities, strengths, and likely COAs when developing their support plan.
Terrain determines the commander's options for designing delays. For example, open and unobstructed terrain makes a delay more difficult because of the major engineer effort required. Rugged, swampy, and wooded terrain help a delay by slowing down the enemy and reducing the amount of engineer effort required. Obstacles should complement terrain.
During the delay, engineers' efforts center on countermobility; however, they also improve the delaying force's mobility and survivability. As units move to their subsequent battle positions, obstacles close routes to the advancing enemy. Units report these obstacles to the higher HQ so the commander knows whether the enemy is able to exploit an open mobility corridor into the next defensive line. Friendly units must quickly pass through obstacles to reach subsequent battle positions. Engineers can expect breaching and other mobility missions to keep withdrawal and supply routes unobstructed. Mobility improvement will continue to the battle positions, in depth.
Delaying commanders attempt to avoid decisive combat because of their forces' size. They must frequently pass units through each other forward, rearward, or laterally to keep them fresh, throw the enemy off balance, and preserve the forces' integrity. As a rule, the delaying force needs to be at least as mobile as the enemy's. This means that engineers must be able to construct significant obstacles and battle positions, in depth. This can require a significant engineer effort to ensure that mobility lanes exist between battle positions.
The maneuver unit maintains flexibility through in-stride breaching and river-crossing operations. A delaying unit may be required to conduct hasty river-crossing operations. The commander and engineer should anticipate the need for breaching equipment and have it available, when necessary. Flexible planning allows the units conducting a river crossing to adapt quickly to changes during execution. Some important features of a flexible plan include¯
Multiple approach routes from battle positions to crossing sites.
Lateral routes between crossing sites.
Alternate crossing sites if threat actions close primary sites.
Crossing equipment held in reserve to replace losses or open alternate sites.
Preplanned EAs to block enemy advances. A delay is most effective when the deception confuses the enemy as to the true intentions of the delaying force. Obstacles must not reveal friendly positions or plans. Concealed, dummy, and expedient obstacles help in deception operations. Deception, therefore, can be considered a force multiplier.
Commanders must use available time effectively so subordinates have enough time to plan and prepare their operations. They must remember that the longer the force delays, the more engineers will need support to sustain the operation.

Withdrawal

Units conduct withdrawals to extract forces from combat, adjust defensive positions, or relocate. Friendly forces voluntarily disengage from the enemy and move rearward. The enemy usually does not pressure withdrawing units. Also, other friendly units do not normally assist in withdrawals. Engineers may emplace obstacles to prevent or slow direct enemy pressure. If the enemy tries to impede the movement of withdrawing forces, engineers must be prepared to reduce obstacles and repair routes. Breaching and river-crossing operations may also be necessary to prevent force isolation and encirclement.

Deception operations are important when protecting friendly forces, and engineers assist in the deception. False defensive positions deceive the enemy about the true location of friendly forces. METT-T determines the method or materials used for deception operations. Since engineer activity is a scarce resource, it is very effective in portraying a false picture. Dummy obstacles, simulated minefields, shallow ditches, and weapons positions can deceive the enemy and aid force survivability. Deception measures also degrade enemy C2 by creating confusion.

If a unit has difficulty breaking with the enemy in a withdrawal, it can request help from a higher level. The assisted withdrawal will be a rearward passage of lines. Engineers from both units coordinate during the planning and execution of the passage. They exchange information on obstacles and routes in the sector. The assisting unit provides mobility support along cleared routes and corridors in its sector for the passing unit.

Engineers must complete clearing operations before the passage begins. The assisting unit also closes the lanes once passage is complete. The passing unit must plan and organize for in-stride breaching and, if necessary, river-crossing operations before initiating the passage of lines. This should ensure responsive mobility operations if the enemy blocks routes during the passage. An unassisted withdrawal occurs when the withdrawing unit is not under enemy pressure. Engineers perform similar missions as an assisted withdrawal except they only use internal assets.

Retirement

A retirement is units not in contact with the enemy moving to the rear area. Engineers plan a retirement as they would a withdrawal. The commander must have the assets available for possible breaching and river-crossing operations. Speed is important; therefore, engineers should focus on mobility for the retiring unit and expect operations such as route clearance and route repair. They can also expect countermobility and survivability missions in support of the rear guard force.

Transition in Retrograde Operations

Detailed planning and execution are key to a successful transition. Planning can be deliberate, which could happen before a maneuver starts. Planning can be rapid, which would occur at the same time as an operation.

Transition to the Offense

Gaining the initiative and employing offensive operations are vital to success. Situations could arise during retrograde operations that could allow a transition to the offense. Obstacle zones and belts allow free maneuver and offensive transitions to occur rapidly. Since the priority of engineer support and task organization during retrograde operations is essentially the same as in the offense, the transition should be smooth. The engineer commander must ensure that the engineer structure provides the necessary physical agility for the maneuver commander.

Transition to the Defense

The maneuver commander and the engineer must plan for transition to the defense. The OPLAN will include a detailed plan for the transition. Engineers must be ready to develop fighting positions to protect the force and emplace obstacles to fix the attacking forces' areas and deny them the ability to quickly overrun positions. Since it is unclear where and when that may occur, planning for the transition to the defense will be less detailed. When changing from retrograde to defense, priority of engineer support shifts from mobility to survivability and countermobility.

Success depends on the force's ability to make this shift rapidly. The initial task organization for the retrograde should not change significantly. The engineer commander must provide additional support for survivability and countermobility when preparing and while transitioning to the defense. Engineers with the maneuver force immediately begin defensive preparations, supported by follow-on engineers. Engineer digging equipment is brought forward quickly to assist. The defense requires extensive engineer Class IV and V materials, which must be ready to move forward in the logistics system. The defense also must retain the flexibility to resume retrograde operations for the offense, if necessary.

DENIAL OPERATIONS

Denial operations have always been a part of war and, on occasion, have attained significant importance. Today, we conduct combat operations according to the laws of war. We take all reasonable precautions to avoid incidental injury to civilians and minimize unnecessary damage to their objects. The laws of war require that denial operations, particularly against civilian facilities and resources, be carefully considered and that execution authority be maintained at the highest levels.

Destroying or removing objects that have a tactical or strategic use (denial targets) could reduce the enemy's benefit from capturing an area. Engineers play a major role in executing denial operations because of their heavy equipment and demolition capabilities. Even though engineers execute certain denial measures, they must not be confused with obstacles. Obstacles are employed to attack the enemy's mobility. A denial target may be an obstacle and can be included in an obstacle plan, but its primary purpose is to deny an immediate benefit to the enemy.

TARGET SELECTION

An effective denial operation focuses on objects with high military value. Whenever possible, they are selected to aggravate enemy weaknesses and limitations. Commanders should ensure that denial targets¯

Disrupt the enemy's logistics-support capabilities.
Prevent the use of local materials, supplies, and equipment to reinforce offensive capabilities.
Require the enemy to divert significant engineer effort for repair.
Restrict mobility. Denial targets should accomplish at least one of these criteria, in a substantial manner, with the least possible amount of damage to civil property.

LIMITATIONS

Denial operations are somewhat restrictive. Only those civilian targets with a clearly identified military value can be destroyed or removed. Looting or excessive destruction is prohibited. Policy states that objects such as churches, medical supplies, and hospitals are protected against destruction unless the enemy uses them for military purposes.

PLANNING

Detailed planning must occur at all levels to implement established denial policy. Coordination between the theater command and the HN governments is important in the policy development process. Operations plans and orders assign denial targets and mission responsibilities at corps and subordinate levels based on this policy. Each corps and division then prepares formal denial plans.

Military supplies and equipment are evacuated when possible. If they cannot be evacuated, they are destroyed so the enemy cannot use them. All available means of transportation, to include civilian vehicles, must be used to save as many supplies and as much equipment as possible. Coordination is essential to ensure that denial operations do not isolate or remove vital support from friendly elements.

PERSPECTIVE:

As Union forces advanced near Yorktown in 1862, the Confederate rear guard set out to delay the Union movement. One commander, BG Gabriel Rains, planted a number of artillery shells in the road with contact fuzes or trip wires. This became the first documented use of contact mines in American history. The tactic worked as Union cavalry became increasingly hesitant to move along the various roads due to the potential of death from the "subterra shells." Subsequently, similar mines were used to supplement coastal fortifications, defend Richmond, and delay Sherman's march to the sea in 1864.

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