3-135. Additional defensive planning considerations for missions in urban and mountainous environments include urban terrain, subterranean threats, mountainous terrain and tunnel and cave complexes. Each of these are described below.
3-136. Infantry forces defend urban areas to defeat an attack, gain time, economize forces, protect infrastructure, protect a populace, and shape conditions for offensive or stability urban operations. Usually two or more purposes apply to urban defense tasks in urban terrain. Defensive urban operations provide leaders opportunities to turn the environment’s characteristics to the advantage of Army forces. Urban areas are ideal for the defense and enhance the combat power of defending units.
3-137. In a built-up area, the defender takes advantage of inherent cover and concealment afforded by urban terrain. Restrictions to the attacker's ability to maneuver and observe are taken into consideration. By using the terrain and fighting from well-prepared and mutually supporting positions, a defending force can delay, block, fix, turn, disrupt, or destroy a much larger attacking force. The defense of a built-up area is organized around key terrain features, buildings, and areas that preserves the integrity of the defense and provide the defender ease of movement. The defender organizes and plans defensive missions by considering OAKOC, fire hazards, and communications restrictions. (Refer to ATTP 3-06.11 for more information.)
Watch the following video to learn more about additional planning considerations.
3-138. The enemy will likely use tunnels and may have the advantage of marked routes and detailed reconnaissance. Because he is able to select ambush positions and withdrawal routes, the defender typically has the element of surprise. A defended position in an underground facility can be very effective in countering enemy subterranean operations. The best underground defensive positions are well protected and canalize the enemy into a killing zone to inflict maximum casualties.
3-139. When moving through tunnels, take great care to avoid booby traps. These are normally deployed near junctions and are often operated by tripwires. Standing water in tunnels provides excellent camouflage for antipersonnel mines and booby traps scattered on likely routes. With the battle above continuing, flooding and cave-ins are highly possible due to the likelihood of artillery barrages and the use of demolitions. Thus, identifying escape routes is essential.
3-140. Chemical defense is a constant concern for Soldiers conducting subterranean operations. In tunnels, Soldiers may encounter chemical warfare agents as well as industrial chemicals in dense concentrations. A chemical agent alarm system, carried by the point man, provides instantaneous warning of the presence of chemical warfare agents. M8 and M9 detection papers also test for the presence of chemical agents. (Refer to ATTP 3-06.11 for more information.
3-141. Defensive tasks in mountainous areas are conducted to resist, defeat, or destroy an enemy attack to support subsequent offensive tasks. Infantry leaders use the defense to withstand an enemy attack while preparing to seize the initiative and develop conditions favorable for transitioning to the offense. During the defense, friendly forces withstand enemy attacks and hold the enemy while preparing to seize the initiative and transition to an attack or to conduct stability tasks. A thorough understanding of the commander's intent is especially critical in the defense, which demands precise integration of all combat power.
3-142. Forces operating in mountainous terrain often possess weapons and equipment more advanced in technology than the enemy does. Knowing this, enemy offensive tactics commonly involve short violent engagements followed by a hasty withdrawal through preplanned routes. They often strike quickly and fight only as long as the advantage of initial surprise is in their favor. Attacks may include direct fires, indirect fires, or IEDs and may be against stationary or moving forces. (Refer to ATTP 3-21.50 for more information.)
TUNNEL AND CAVE COMPLEXES
3-143. Tunnel and complexes may be interconnected with other tunnels and caves, concealed by trapdoors or blocked dirt passages that are up to three or four feet thick. Secret passages are usually known only to selected personnel and are used mainly in emergencies. Tunnels and caves may be interconnected by much longer passages through which relatively large bodies of men may be transferred from one area to another. The connectivity of these systems often allows the enemy to move unnoticed from one area to another, eluding friendly forces.
3-144. Characteristic of a typical tunnel or cave complex is normally superb camouflage, conceal entrances, exits and camouflage bunkers. Within the tunnel and cave complex itself, side tunnels may be concealed, trapdoors are often hidden and dead-end tunnels or caves are used to confuse the attackers. Airshafts are usually spaced at intervals throughout a tunnel or cave system. In many instances, the first indication of a tunnel or cave complex comes from direct fire received from a concealed bunker. Spoils from the tunnel or cave system may be distributed over a wide area, giving clues to its existence. (Refer to ATTP 3-21.50 for more information).