The Way Ahead is Down: The Case for Underground Defense
Introduction: A Role Reversal
To defeat China in armed conflict, the US military must embrace underground defenses. Military history repeatedly demonstrates that subterranean defenses are effective counters to superior firepower. The volcanic caves of Iwo Jima, the communist Vietnamese tunnels of Cu Chi, and the mountain hideouts in Afghanistan are all infamous in US military lore.
In each of these cases, American forces are always on the offense thanks to superior logistics, intelligence collection, and firepower. As such, current US military planners approach underground combat strictly from an attacker’s perspective, consistent with the American Way of War. There are specialized units, doctrine, and training to penetrate and clear underground facilities, but no reciprocal efforts dedicated to constructing and defending underground fortifications. There is an assumption that US forces will always be the side to force their adversaries underground. In the Pacific, this presumed position of strength is eroding daily as China expands its long-range strike capabilities. In response, the US military should invest in tactical underground defenses in the Pacific to harden allies, deter aggression, and hold key terrain in armed conflict.
The Limits of Dispersion
Any conflict with China in vicinity of the first island chain places the US at an extreme disadvantage. The People’s Liberation Army (PLA) enjoys shorter supply lines supporting an increasingly more sophisticated anti-access, area denial battle system. The PLA A2/AD system includes outposts of artificial islands, hundreds of maritime militia vessels, long-range missiles, fifth-generation fighters, and a growing navy built around modern aircraft carriers. This impressive constellation of sensors and shooters was developed to prevent the US from concentrating combat power, as demonstrated in the Gulf War.
To survive inside the PLA’s weapon engagement zone (WEZ), the US military is counting on dispersion. The Marine Corps divested heavier equipment, such as tanks and military bridges, in support of their Expeditionary Advanced Base Operations concept, which favors lighter, more mobile forces. The Air Force Agile Combat Employment concept eschews large, permanent air bases for smaller, temporary bases. The Army is creating mobile combat teams, trading their heavy joint light tactical vehicles (JLTVs) for lighter infantry squad vehicles built on a pick-up truck frame. These concepts are rational tactical solutions, but present serious vulnerabilities.
Dispersion has its limits. Land, air, and sea forces will always be tied to fixed logistical nodes. Tenuous access, basing, and overflight (ABO) authorities may also limit which countries US forces can operate in. Even with ABO, many Pacific countries, most notably the Philippines lack widespread infrastructure to support military sustainment such as suitable bridges, deep water ports, and C-17 capable runways. In addition to these inconvenient realities is the underlying assumption that US forces can move faster than they can be targeted. China’s investments in space-based surveillance, drones, and precision fires greatly increase their ability to target even mobile forces. In Ukraine, the proliferation of loitering munitions and drones spotting for artillery has forced both sides to dig into WWI style trenches. On the future battlefield, dispersion may be necessary, but it is not sufficient.
The Case for Underground Defenses: Harden, Deter, and Defend
The solution to operating inside the PLA WEZ is simple yet historically un-American—go underground. Underground defenses will deter aggression, strengthen allies, and in conflict, retain key terrain.
Subterranean defenses are an affordable, accessible, and effective tool to strengthen allies and partners. Building tunnels and bunkers does not require expensive or exquisite capabilities. Relatively poor fighting forces such as Hamas and ISIS built tunnels with little more than hand tools. Thus, the US should be actively encouraging and involved with tunnel building efforts with its allies and partners. By being directly involved with tunnel construction, the US can ensure that their specifications will allow US personnel and equipment to operate in them. Host nation underground defenses could also aid existing concepts of resistance by providing secure lines of communication for hastily activated irregular forces. Tunnels protect defenders from surveillance as well as kinetic attacks, which allows them to better reposition units and weapons systems. While partner nations should take the lead in their underground defenses, the US should assist and subsidize tunnel construction as a tangible commitment to mutual defense.
Underground defenses are a significant deterrent. Strategic weapons are often protected in underground facilities to provide a second-strike capability. If an aggressor nation is unlikely to destroy their enemy’s nuclear silo in the first volley they are unwilling to accept the inevitable retaliation. The same logic applies to tactical forces. For example, the miles of North Korean tunnels are a serious deterrent to any pre-emptive attacks on the troublesome hermit kingdom. Underground defenses would also allow the US to better protect and forward stage critical equipment such as the Army’s Mid Range Capability and Long Range Hypersonic Weapons in the Multi Domain Task Forces.
In the event deterrence fails, underground defenses are the most proven way to retain key terrain until a relief force can be organized. As stated earlier, the US has learned time and again how effective robust underground defensive networks are at delaying attackers. In early 1945, the Battle of Iwo Jima proved the deadly efficacy of a subterranean defense to blunt a superior attacking force employing overwhelming firepower. The US bombarded the tiny volcanic island with over 20,000 tons of explosives for over nine months, and analysts were confident it could be taken in just 7 days. However, the fight dragged on for over five bloody weeks, resulting in over 26,000 American casualties. Two Japanese soldiers held out for four additional years, demonstrating just how robust the tunnels and their defenders were.
While the defenses of Iwo Jima significantly delayed US forces, it did not stop Imperial Japan’s eventual defeat. As explained by Clausewitz, defense may be the stronger form of warfare, but it has a negative aim. Any operational approach employing static defenses must have a credible mobile striking force. Even with their operational limitations, underground defenses can be a simple, effective component in frustrating PLA aggression in the Pacific.
The Way Ahead is Down
The US military must develop the ability to build and fight from underground facilities. These underground defenses must include more than solitary fighting positions. Like the defenses of Iwo Jima, they should be a defensive network of bunkers, communication tunnels, supply caches, hospitals, and headquarters. Sprawling underground defensive networks may sound expensive at first, but they could be scalable and iterative. Defensive networks could begin as simple fighting positions and then steadily evolve to include logistical and command capabilities. Currently, combat engineers lack any doctrine, training, or equipment to build even hasty underground shelters. If guerrilla fighters such as the Vietnamese communists and Hamas can build formidable subterranean defenses with little more than hand tools, then surely the US can.
In addition to construction capabilities, the US must train its soldiers and marines how to operate underground on the defensive. The tactics of tunnel defense vary greatly from maneuvering in the open. Current underground combat training is focused from an attacker’s perspective, with a heavy emphasis on breaching. Tunnel defenders must fight differently. They must be patient, acclimatized to their underground environment, and able to operate in much smaller teams. The Vietnamese tunnel defenders of Cu Chi were trained to utilize grenades, trap doors, and escape shafts to delay much larger American formations.
Tactical underground defensive capabilities are a simple but currently unavailable asymmetric advantage to US forces operating in the Pacific. As the age of American overmatch in the skies becomes less certain, senior military planners must learn the lessons from their historical adversaries, who recognized the inherent strength of subterranean defenses.
