The most pressing question for Taiwan is its defensive strategy: How would the Republic of China (Taiwan) defend itself alone—even temporarily? The solution could be an operational approach focused on attrition and achieved tactically through subterranean operations within Taiwan’s urban and mountain environments.
What follows are some planning considerations for course-of-action development and wargaming specific to subterranean urban and subterranean mountain warfare operations. The baseline assumption is a worst-case scenario: an initial and successful invasion of Taiwan by the People’s Liberation Army Navy (PLAN) carried out through multiple simultaneous amphibious assaults across Taiwan’s coastline. This would establish the conditions for PLA follow-on forces to advance inland.1 Any advance beyond the necessary beachheads would lead directly into Taiwan’s urban and mountain environments.
This natural and man-made geography informs the second assumption: Having failed to prevent the PLA from projecting power ashore, Taiwanese forces would conduct a mobile defense. These defensive operations could take advantage of the unique delaying characteristics of the subterranean layers of both the urban and mountain environments. The final assumption is that, at least initially, Taiwan would have to go it alone—no forces from the United States or other nations would intervene in the short term—and the tactical use of nuclear weapons is likewise assumed ruled out.
Through the lens of the seven warfighting functions, it is possible to identify some of the tactics, techniques, and procedures involving subterranean urban and mountain warfare operations that could significantly attrit an invading PLA force.2
Playing Urban Defense
Urban defensive operations could provide Taiwan’s armed forces the ability to establish numerous delay lines by which they would defend, reinforce, attack, withdraw, and slow what most likely would be a numerically superior PLA force.3 This approach could significantly attrit PLA forces while the Taiwanese military maintains the initiative. A potential, arguably desperate, result could be that PLA forces could mass fires to target urban areas in an attempt to regain the initiative. Despite a terrible cost, such an action could still work to Taiwan’s advantage. The reasons are twofold: First, massive devastation of urban areas creates significant rubble and, by default, numerous defensive positions on the surface layer. This would most likely require PLA forces to clear and hold both surface and subterranean layers as they advance, a laborious process.4 Second, use of massed fires could turn international public opinion—an essential element of the information environment—against China, with images of devastation and its corresponding effects on the civilian population.
Future success requires the immediate implementation of those measures that would enable Taiwan to conduct a deliberate defense if an invasion were to occur.5 This might seem obvious, but if the PLA succeeds in establishing forced beachheads, a successful defense will require an underlying strategy that cannot rely on intervention by outside forces.
Is Taiwan Prepared?
The urban environment consists of three layers: the surface (street), the supersurface (rooftops and the air above), and the subsurface (subterranean).
The latter consists of underground facilities—subway systems, parking garages, shopping malls, restaurants, and storage facilities, etc.6 Taiwan also has an elaborate network of subterranean tunnels, some new, but many in various states of disrepair that would require extensive renovation to be used for defensive purposes.
And adjacent to Taiwan are 150 smaller islands, some with significant urban areas.7 These heavily fortified islands could serve as a means to disrupt and grind down any PLA invasion force. By default, Taiwanese forces on these islands are not only defensive in nature but also arguably “forward deployed” and could conduct maritime operations such as intelligence, surveillance, and reconnaissance, deployment of special forces, and small boat operations using fast-attack craft.
For example, roughly six miles from the Fujian region of China lie the Matsu Islands, which are heavily fortified with underground tunnels and bunkers. Also roughly six miles from mainland China are the Kinmen Islands, another island group fortified with numerous bomb shelters. The Kinmens’ main island has more than 1,000 subterranean facilities, many connected via tunnels with ammunition storage capabilities.8
The answer to the question of Taiwan’s preparedness for subterranean urban operations is mixed. The requirements, however, are not. An effective operational approach involving tactics, techniques, and procedures focused on attrition will require an immediate investment in construction or repair of subterranean facilities, training of personnel to conduct operations independently—decentralized control—and a dramatic increase in ammunition production.9 It is important that Taiwan recognizes this and continues to repair existing subterranean facilities, constructs new ones, and increases training for subterranean operations. In addition, Taiwan should continue developing plans for the coordination of civil-military operations, through training events that emphasize resiliency, preparedness, and evacuation measures for the civilian population to designated air raid shelters.10
Keelung is a mountainous port city on Taiwan’s northern coast. It has 700 bomb shelters, bunkers, and tunnels—in conditions ranging from poor to functional—with a mixture of 19th century and relatively modern ones, the latter mostly built by Imperial Japanese forces before and during World War II.11
Regardless, renovating these existing structures and linking them would improve the viability of conducting subterranean mountain warfare operations. Consider Chiashan Air Force Base, also referred to as Optimal Mountain. It protects as many as 200 sophisticated fixed-wing aircraft, including F-16V Vipers. This facility has multiple multistoried horizontal and vertical layers; steel blast doors; and other infrastructure that supports command and control. It demonstrates Taiwan’s sophistication and capability in building subterranean facilities as well as the scale of construction requirements.12 Additional examples exist, such as the Hengshan Defense Command at the Taitung Air Force Base, the Air Defense Command Center near Taida University in Taipei, and others across the main island, all constructed deep within various mountain ranges.
Is Taiwan prepared for subterranean mountain warfare operations? Here, too, the answer is mixed, and improvement will require additional investment and training. Any training initiatives for subterranean mountain warfare will need to emphasize decentralized control that enables defensive operations that inflict the most material and personnel punishment on PLA forces.
Warfighting Functions
The seven mutually supporting warfighting functions require full integration into an operational approach focused on attrition. All can be tested through carefully designed wargames, and below is a brief list of planning considerations for wargaming.
Command and control. An approach emphasizing subterranean operations and their associated TTPs would require dispersed command elements throughout Taiwan. Success would require decentralized operations conducted through decentralized control, with subordinate units operating independently. This would require an increase in trust from headquarters elements, which would require small-unit training that emphasizes mission command.13
Fires. Any effective attrition of PLA forces would require a combination of urban and mountain subterranean operations and indirect fires, such as artillery from hardened facilities, and—given the nature of the urban and mountain environments—mortar systems, in particular, with their high-angle, plunging fires. Planning should include deliberate targeting—preparing ahead of time to attack known and fixed targets. Hardened subterranean facilities would provide protection for wheeled or tracked artillery, rocket, and air-defense systems as well as a concealed means of maneuver. Planning should also allow for widespread use of unmanned ground vehicles (UGVs) and aerial systems (UASs) for observation and targeting of advancing PLA forces.
Force protection. Hardened subterranean facilities could offer protection from PLA artillery, mortars, naval surface fire support, offensive air support, and guided missiles, etc. Such facilities also would provide the means, either purpose-built or adapted, to protect command-and-control assets and personnel who will assist subordinate units in conducting decentralized operations. Force-protection measures should include establishment of defensive minefields in the urban and mountain environments as well as other obstacles that could delay PLA forces by maximizing their exposure to indirect fires.
Information. Activities within the information environment should seek to exploit any successes of subterranean operations against PLA forces.14 This would require messaging with a consistent narrative and images that underscore Taiwan’s defensive effort against a recidivist superpower. Recent conflicts from Afghanistan, Iraq, and the Philippines to the ongoing conflicts in Ukraine and Gaza underscore the importance of aligning operations with messaging and videos. UASs, combat footage, the internet, mobile phones, social media, etc., must all contribute. The objective would be to provide continual updates regarding the plight of the population and maintain a message that Taiwan is defending its sovereignty, its right to self-determination, and individual rights while upholding international law.
Intelligence. Intelligence activities should start with identifying long forgotten historical subterranean facilities that could be converted to military use and using geospatial intelligence capabilities to prepare maps that indicate the best means of maneuver through these for Taiwanese forces against the PLA. Intelligence collection and assessment requirements should also include an understanding of PLA capabilities for fighting underground, derived from PLA doctrinal publications and other material that share the most relevant lessons the PLA has drawn from urban battles in the past few decades. During any actual operations, Taiwan should use UASs for observation and targeting, employing assessments developed through wargaming to expedite execution of specific courses of action against the PLA in Taiwan.
Logistics. Existing and newly built hardened subterranean urban and mountain facilities will be force-enablers during an invasion. The need to acquire, manufacture, and stockpile all classes of supply in such facilities—now—cannot be overstated. The defensive approach proposed here demands it. The continuous availability of supplies through extensive stockpiling will sustain operating tempo. The ongoing conflict in Ukraine underscores that need through the unexpected and extravagant ammunition usage rates, particularly for indirect-fire systems. That conflict also has underscored the need for spare parts, such as replacement barrels for artillery systems as well as hardened and dispersed repair facilities.
Maneuver. Subterranean urban and mountain layers provide the means to maneuver ground combat element and combat support service echelons large and small. This could include armor and trucks to support operations on all layers—surface, subsurface, and supersurface. For example, an elaborate network of tunnels could assist Taiwan in maintaining operating tempo and the initiative. The tunnels could permit creation of strongpoints at will and maneuver behind advancing enemy forces, as well as constant engagement of PLA formations with small arms, antitank guided missiles, and indirect fires from artillery, mortar, and rocket systems within hardened facilities, guided by UASs—not to mention the potential use of UAS swarms for continuous observation and targeting.
Going to Ground
History has demonstrated the effectiveness of underground military operations—planned or improvised—against an invading military.15 For Taiwan to win the urban fight will require a tactical focus on the subsurface layer. The challenge, however, is defining what winning means. Given the assumption that Taiwan will conduct independent operations without external support, winning the urban fight could mean that Taiwan renders any PLA offensive operations extraordinarily costly, exhausting PLA resources and limiting its ability to operate. This could render China vulnerable to external actions by Taiwan’s friends elsewhere within the Indo-Pacific region.
Accomplishing this would require Taiwan to continue to develop new and renovate existing underground facilities. Evaluating this assumption through wargaming is a necessity. “Winning the urban fight” does not necessarily equate to outright destruction of PLA forces. Defensive operations focused on attrition would enable Taiwan to trade space for time, again at a significant cost to the PLA. Becoming ready to do this, too, requires further wargaming, but any significant delay imposed on PLA offensive operations could change the international dynamic, allowing third-party countries to decide to become involved and reinforce Taiwan or push diplomatic solutions. Taiwan’s ability to delay and significantly attrit a Chinese invasion force from below the ground could be the determining factors for winning an urban fight.
1. Elsa B. Kania and Ian Burns McCaslin, The PLA’s Evolving Outlook on Urban Warfare: Learning, Training, and Implications for Taiwan, Military Learning and Future of War Series (Washington, DC: Institute for the Study of War, April 2022).
2. For a brief overview of strategic planning considerations and potential political and military outcomes regarding an invasion of Taiwan, see Maj Andrew Krebs, USMC, “Answering the Taiwan Question: What Is the United States’ Strategic Interest?” The Marine Corps Gazette 107, no. 2 (February 2023): 96–100.
3. A delay line is a specified line forward of which a force must slow the enemy for a specified time or until a specified event. Marine Corps Reference Publication (MCRP) 1-10.2: Marine Corps Supplement to the DOD Dictionary of Military and Associated Terms (Washington, DC: Headquarters U.S. Marine Corps, 12 February 2018), II-23.
4. For a brief overview of the PLA’s understanding of tactical urban offensive and urban defensive operations, see Army Techniques Publication No. 7-100.3: Chinese Tactics (Washington, DC: Headquarters, Department of the Army, 24 November 2021), 7-20 to 7-22 and 8-28 to 8-29.
5. See “deliberate defense,” MCRP 1-10.2: Marine Corps Supplement, II-23.
6. Army Techniques Publication 3-21.5, Subterranean Operations (Washington, DC: Headquarters, Department of the Army, 1 November 2019).
7. Benedict Brook, “Islands Just Kilometres from China That Are ‘Vulnerable’ to Invasion,” The Courier Mail, 1 December 2020.
8. Many shelters are now tourist attractions, and these require immediate repurposing for military operations.
9. See “decentralized control,” MCRP 1-10.2: Marine Corps Supplement, II-21.
10. William Yang Taipei, “How the Ukraine War Has Pushed Taiwan to Better Prepare for Crises,” DNYUZ, 31 August 2022.
11. Damine Cave and Amy Chang Chien, “Taiwan’s Bomb Shelters: ‘A Space for Life. And a Space for Death,’” The New York Times, 6 November 2022.
12. Emma Helfrich, “Extremely Rare Photos Inside Taiwan’s Underground Fighter Jet Caves,” The War Zone (26 July 2022).
13. See “mission command,” Chairman of the Joint Chiefs of Staff, DOD Dictionary of Military and Associated Terms (Washington, DC: The Joint Staff, July 2024), 124.
14. Marine Corps Doctrinal Publication 8: Information (Washington, DC: Headquarters U.S. Marine Corps, 22 June 2022), 1-7.
15. Numerous historical examples demonstrate the effectiveness of conducting a delaying action within the subterranean layer of an urban environment. One of the most familiar is the Battle of Stalingrad (August 1942–February 1943). Red Army (Soviet Union) units conducted urban operations, including extensive operations in the Soviet city’s subsurface layer, which delayed the invading Wehrmacht long enough to prevent the city’s surrender.This delaying action enabled the planning, logistical build-up, and successful execution of Operation Uranus, a massive pincer movement by the Red Army that encircled Stalingrad, prevented the resupply of Wehrmacht units within, and ultimately led to their defeat.
Another excellent example of how military forces can use the subterranean layer during mountain warfare operations occurred in the early stages of the Korean War (1950) when the North Korean People’s Army executed a delaying action throughout a series of hills surrounding the South Korean capital of Seoul. The NKPA’s use of this dominant terrain feature, including caves and an extensive defensive network, to delay the advance of U.S. forces prior to Operation Chromite, the Inchon-Seoul campaign.
A more recent example is the Ukrainian Army’s attacks on the Russian Army during Russia’s assault on Mariupol (February 2022–May 2022). Units of the Russian Army initially secured the surface and supersurface layers of the city, but remaining Ukrainian forces retreated to the subsurface (subterranean) layer, specifically the Azovstal Steel Works facility. The Russian Army, through arguably a combination of limited personnel, training, and will, did not conduct subsurface operations there, instead, expending significant quantities of munitions in an attempt to dislodge and defeat the Ukrainian defenders.While Ukrainian forces eventually surrendered because of low ammunition stocks, extensive casualties, and a significant number of civilians to care for, the delaying action that resulted required a large number of Russian forces to remain in the city, forces that could have been used on other fronts.See Michael Schwirtz, “Last Stand at Azovstal: Inside the Siege that Shaped the Ukraine War,” The New York Times, 27 July 2022.
