Fortification and Strategy, Part II

This is the second of two companion papers on the return of a fortified disposition to the center of strategy. The first paper treated fortification as a movement-slowing strategic complex rather than a simple barrier, traced the long attenuation of that disposition in Western military practice, and examined the first contemporary trend: the return of guarded, closable cities through dual-use infrastructure and regulation. It closed with the observation that fortress design is again becoming part of the officer’s expected competence, even when it arrives disguised as a planning code or a latent structure in ordinary fabric.

The present paper takes up the other two visible trends—the revival of large-scale static defensive systems and the renaissance of tunnelling and subterranean engineering—and then draws the argument together. The underlying drivers are the same. Trained manpower and high-end platforms are scarce; surveillance is persistent; open movement is expensive. Prepared ground is once more a rational substitute for labour and for vulnerable platforms. What changes here is the scale and the medium. Where the first paper showed how the city may harden from within, this one explains how states are relearning how to build delay-and-attrition systems across landscapes, and how to go beneath a surface that sensors and cheap precision munitions have made increasingly lethal.

The strategic and economic logic is old. A position that can be built once, lightly manned and amortised over decades is the same substitution of capital for scarce labour that once made medieval castles and Vauban’s pré carré rational. When the alternative is a platform that cannot hide, prepared ground recovers its value. Ian Hogg asked that the fortress engineer be remembered. The first place to look for his return is the large static system.

The Revival of Large-Scale Static Fortifications

The economic scale of such choices is often understated, and historical judgements are frequently unfair. People still cite the Maginot Line as a costly white elephant. The real failure in 1940 was not that the Line, conceived as a shield, failed to perform its defensive function. The sectors the Germans actually assaulted held firmly, even when manned by lower-quality troops. The decisive weakness lay in the French Army’s inability to act as an effective sword—specifically, its lack of sufficient mobile, well-equipped forces capable of fighting a war of manoeuvre in the unfortified gap through Belgium and the Ardennes. In short, the fortifications themselves largely did what they were designed to do; it was the broader operational and force-structure concept surrounding them that collapsed.[1]

The lesson the victors drew was not that permanent works were obsolete. On the contrary, their subsequent wartime investment in permanent defensive works imposed an even greater burden on the economy, many times exceeding the cost of the Maginot Line. The Atlantic Wall alone consumed approximately 17 million cubic metres of concrete and 1.2 million tonnes of steel—equivalent to just over 5 per cent of annual German steel production—at a cost of 3.7 billion Reichsmarks for the French section alone.

Parallel programmes, notably the massive flak towers in Berlin, Hamburg and Vienna together with the wider network of air-defence bunkers, absorbed further large quantities of cement, steel and labour at a time when these resources were also required for U-boat pens, industrial construction and the field army. Taken together, these projects represented one of the largest diversions of capital, materials and manpower into permanent defensive works of any combatant in the Second World War.[2]

Contemporary experience is already demonstrating the continuing operational value of such systems. Ukraine’s Fortress Belt in Donetsk Oblast provides the clearest current illustration. Successive Institute for the Study of War assessments through 2025–2026 show how this chain of reinforced cities and supporting fortifications—developed and improved over more than a decade—has repeatedly frustrated Russian breakthrough attempts under conditions of persistent drone and artillery observation. The density of prepared positions, mutually supporting strongpoints and protected logistics routes has forced attackers into costly, incremental advances rather than rapid operational manoeuvre. Analyses linking fortification efforts to acute manpower and munitions shortages reinforce the underlying economic logic: when trained personnel and precision munitions are scarce, prepared ground multiplies combat power.[3]

Along NATO’s eastern flank the same logic is being applied at scale. The Baltic Defence Line, Finnish border obstacles and Poland’s East Shield represent the most significant defensive hardening in Europe since the Cold War. These are not simple Maginot revivals. They are multi-domain adaptations designed to delay, channel and attrit rather than to create an impermeable barrier, buying time for mobile reserves and allied reinforcement. The aim is to ensure that any aggression is neither quick nor cheap.[4]

Historical models from the mid-twentieth century remain directly relevant. Norway’s Fjell Fortress, built into a mountain to defend Bergen, is particularly instructive. Its centrepiece was a triple 28 cm gun turret taken from the German battlecruiser Gneisenau. Designed as an epitome of naval mobility, the ship had become a liability by 1942 once air power and reconnaissance made surface operations too dangerous. The turret was removed and emplaced in a heavily protected fixed position, converting a high-value mobile asset into a resilient coastal battery.

Fjell belonged to the same family of mid-twentieth-century fixed fortifications as the Atlantic Wall—the extensive system of concrete bunkers, casemates, gun emplacements, command posts and obstacle belts constructed along the western European coast. Both reflected the calculation of high fixed capital investment in exchange for lower recurrent manpower needs and the ability to impose disproportionate costs on an attacker. Similar logic appeared in Japanese island defences and in elements of Soviet permanent fortifications.[5]

These forms have not lost their purpose; they require selective adaptation rather than abandonment. The Atlantic Wall failed in 1944 less because fixed fortifications were inherently obsolete than because it was incomplete, unevenly manned, and faced an opponent with overwhelming air and naval superiority plus the ability to choose the time and place of assault. Where the defences were mature and properly integrated with mobile reserves, they imposed serious costs.

Today’s equivalent of the Gneisenau problem is the vulnerability of advanced aircraft, ships and armoured formations to persistent sensors and massed precision attack. Networks of hardened positions, underground complexes and dual-use structures, lightly manned and equipped with modern sensors, remote weapon stations and directed-energy systems, perform the same force-multiplication and survivability function.

Additive manufacturing is accelerating this adaptation. On-site 3D concrete printing allows relatively rapid construction of bespoke bunkers, firing positions and protective barriers with reduced labour and logistical demands. Portable systems can produce walls and small fortifications in hours rather than days, using local materials and enabling designs tailored to specific terrain and threats. Combined with modular techniques, the technology updates the mid-twentieth-century template, making dense networks of hardened positions faster and more flexible to create.[6]

What additive manufacturing does for the surface, faster digging is beginning to do beneath it.

The Renaissance of Tunnelling and Subterranean Engineering

The demand for overhead cover and the revival of siegecraft discussed in the previous section lead naturally to the underground domain. When surface movement and surface structures become highly vulnerable, the logical response is to go beneath them. The most transformative trend in contemporary fortification is therefore the resurgence of tunnelling for both deep protection and lateral movement. Surface facilities and open movement are increasingly exposed to persistent surveillance and cheap precision attack; the subterranean realm restores a measure of sanctuary and the possibility of surprise.

This is not a new discovery. Tunnelling has been an integral part of warfare for millennia. In the eighth century BCE, King Hezekiah of Judah ordered the construction of a long water tunnel beneath Jerusalem to secure the city’s supply during an expected Assyrian siege. Sixteen centuries later, during the 1546–47 siege of St Andrews Castle in Scotland, both attackers and defenders cut mines and countermines through solid rock; the tunnels survive today as one of the best-preserved illustrations of early-modern subterranean siegecraft. By the age of Vauban, advanced bastioned fortresses routinely incorporated counterscarp galleries, listening posts and pre-planned countermine networks. The same logic reached industrial scale in the First World War, when elaborate tunnel systems under the Western Front supported listening, movement, explosive attack and protection.[7]

The relative absence of serious tunnelling from Western military conversation in recent decades is therefore the historical anomaly, not the current return. For much of the late twentieth and early twenty-first centuries, air superiority, precision strike and an emphasis on rapid manoeuvre pushed subterranean warfare to the margins of doctrine. That period of neglect is ending.[8]

Ukraine’s experience since 2022 has made the practical value of subterranean systems impossible to ignore. Simple trench lines have evolved into elaborate networks of deep dugouts, covered communication trenches, underground command posts, medical stations and ammunition storage. Under persistent drone observation and massed artillery, forces on both sides have continually improved depth, overhead cover and interconnectivity. The ability to dig, reinforce and operate underground has become a central determinant of endurance and combat effectiveness.[9]

The sophistication now visible in Ukrainian and Russian digging is not only a matter of depth. Recent comparative field trials of three entrenching systems make the same point in miniature. A traditional linear trench with little overhead cover and little concealment performed worst: soldiers felt (and, in practice, were) highly exposed to drones. A conventional trench improved with substantial camouflage and netting produced a different result. Troops felt reasonably secure against the air threat while retaining the great advantage of being able to see outwards.

A fully subterranean layout with complete overhead cover scored higher on felt security, but at a cost. Outward observation was harder, situational awareness declined, and the spoil and other signatures of construction were difficult to hide from an enemy who was himself watching from above. Protection without a view did not automatically produce endurance.

The implication is that a fortification is not simply a lump of concrete in a hole. It is an integrated system of protection, concealment, sensors, communications and psychology. Camouflage and landscaping are not optional extras; they are part of the work. The purpose of prepared ground is to slow the accumulation of stress so that soldiers can last. That requires cover, but it also requires the ability to observe, to communicate and to feel that the position is a fighting instrument rather than a tomb.

Designers who forget the last of these will build shelters that are technically hard and operationally brittle. The fortress engineer is returning; the requirement is that he be intelligent about more than thickness of overhead cover.

Russia has pursued parallel protective developments while also exploiting existing underground infrastructure for offensive purposes. In at least two significant instances—during the fighting for Avdiivka in 2024 and later near Sudzha in the Kursk region in 2025—Russian forces used pipelines and related subterranean routes to infiltrate and attack from unexpected directions. Mines and other forgotten networks have been similarly employed. An important and under-discussed enabler is archival intelligence. Because Soviet-era infrastructure records were highly centralised, detailed maps and plans of mines, tunnels, pipelines and sewers remain available in Moscow. These holdings are combined with modern mapping software to produce three-dimensional models that reveal how existing systems can be used for infiltration or linked by short lateral tunnels.[10]

This capability rests on an exceptionally deep historical foundation. Soviet underground military infrastructure was among the most extensive of the Cold War. Deep leadership bunkers were built around Moscow and in the Urals; strategic missile forces relied on hardened silos and connecting tunnels; critical industries were placed inside mountains; and the Moscow Metro itself was designed with dual-use deep stations. The scale of excavation and the geological data retained in central archives created a lasting subterranean knowledge base that contemporary Russian forces continue to draw upon.

Hamas’s tunnel network in Gaza illustrates both the tactical utility and the economic asymmetry of subterranean warfare. Estimates of its length run to several hundred kilometres, with construction costs assessed in the high hundreds of millions to over one billion dollars. For Hamas this was a major but manageable investment that delivered protected movement, command nodes and the ability to survive intensive assault. Israel has spent decades and far larger sums on counter-tunnel capabilities, including specialised units, detection technologies and a sophisticated border barrier. The disparity is instructive: relatively modest expenditure by the tunnel-builder can compel the opponent to invest disproportionately in detection, clearance and specialised forces—an enduring logic of siegecraft.[11]

Iran’s mountain complexes take the same logic to a national-strategic level. Deeply buried missile facilities support a doctrine of survivable, prolonged strike capacity aimed at attriting an adversary’s high-value munitions and raising the cost of sustained precision campaigns. The confrontation can be read as a contest between two technologies—precision strike from the air and deep subterranean hardening—with other systems playing supporting roles. The underground approach possesses clear advantages: once constructed it is relatively cheap to maintain, difficult to destroy with certainty, and capable of preserving retaliatory capacity under intensive attack.[12]

North Korea and China demonstrate the possibilities at still greater scale. Between the 1970s and 1990, four major North Korean invasion tunnels were discovered under the Demilitarised Zone; some were assessed as capable of passing 30,000 troops per hour. Although decades old, they remain a striking indication of what determined underground effort can achieve. China’s programme is of a different order, reflecting greater resources and a historic propensity for monumental engineering. The so-called ‘Underground Great Wall’, a network of tunnels reported to extend for thousands of kilometres, protects and moves elements of the land-based missile force. The same strategic culture is visible in the creation of fortified artificial islands in the South China Sea. Both cases show that subterranean and large-scale fortification remain live options for states prioritising survivability and depth.[13]

Modern technology has significantly lowered the barriers. Contemporary tunnel-boring machines range from micro-machines roughly one metre in diameter to giants exceeding fifteen metres, with advance rates exceeding 700 metres per week in favourable rock. Greater automation and continuous lining have reduced manpower needs. Micro-tunnelling—compact, often remote-controlled machines for small-diameter drives—enables rapid, low-signature construction of connecting galleries and utility tunnels with minimal surface disruption. Advances in blast engineering allow more controlled excavation and more stable caverns. Combined with robotics, these methods make protected command nodes, storage, power generation and covered movement routes feasible at scales and speeds previously impractical.[14]

The same capabilities open speculative but increasingly plausible offensive possibilities. Imagine an adversary in a heavily fortified position inside a densely populated urban area, where massive surface assault or sustained bombing would carry unacceptable political and humanitarian costs. An operation might instead manoeuvre a one-metre-class micro-tunnelling machine through existing underground infrastructure, drive two approaches (one a decoy), blast a small chamber as a jumping-off point, map the installation with further micro-tunnels, and finally introduce a narrow gallery into a critical void. A robot originally designed for extracting earthquake victims through rubble could then place charges, allowing the position to be imploded from within. In effect this is St Andrews with machines.

This sequence nowadays remains a composite of existing technologies rather than demonstrated practice, yet it illustrates how micro-tunnelling, precision blasting and robotics could alter the balance of urban combat by offering a means of reducing even well-protected targets while limiting surface destruction.

Significant challenges remain—detection, blast propagation, ventilation, water management and the psychological effects of prolonged subterranean operations. These are solvable engineering and training problems rather than fundamental barriers. Professional military interest is rising accordingly.

Much recent attention has focused on the military potential of Starlink and similar low-Earth-orbit constellations, whose performance in Ukraine has underlined their strategic importance.[15] In the longer view, however, it may be Elon Musk’s other venture—the Boring Company—that proves more consequential for the character of warfare. The capacity to dig faster, cheaper and more flexibly than ever before is restoring the underground as a decisive domain. Strategists would do well to look down as well as up.

Conclusion

Ian Hogg closed his history by observing that recent fortifications were usually less interesting, architecturally and aesthetically, than the great works of earlier ages, and that the fortress and the fortress engineer deserved to be remembered. The evidence assembled here suggests that both halves of that judgement are now being revised. Fortifications are once again becoming more interesting—sometimes starkly visible, more often camouflaged within ordinary urban and infrastructural form—and the fortress engineer is returning to a central place in the conduct of strategy.

Fortified urbanism, the revival of sophisticated static systems, and the renaissance of tunnelling are already visible in current conflicts and force planning. They are driven by battlefield transparency, the economics of advanced platforms, manpower constraints, and the rising cost of unprotected manoeuvre. Fortification has always involved substituting capital for labour under scarcity. When the relative prices of trained manpower and high-end platforms rise, prepared positions regain rationality. Dual-use design and modern construction techniques further reduce the opportunity cost relative to earlier pure military works. The longer historical and economic frame developed across these papers explains why the present shift is best understood as a return to a more normal fortified disposition rather than a temporary expedient.

The reinvention of siegecraft is part of the same movement. New construction methods—additive manufacturing, micro-tunnelling, more automated engineering systems, and the possible return of rapidly assemblable surface fortifications—are making the deliberate preparation of ground faster and more flexible than it has been for generations. These technical developments matter, yet they are only instruments. Connecting military action to desired political results has never been solely a matter of aligning weapons with targets. Economy, logistics, culture, popular endurance, and even aesthetics all exert real purchase on the problem. Wars are not won by brilliant manoeuvres and superior weapons alone.

This essay has not addressed the legal and ethical dimensions of the subject, notably the Additional Protocols to the Geneva Conventions concerning protected places and the protection of civilians in fortified or besieged environments. The omission is deliberate: the paper is already long, the author’s primary interests and competence lie elsewhere, and ultimately—as Stanley Baldwin observed in a different context—when a nation believes its back is to the wall it will tend to use whatever means are available to it. Law and ethical norms are important; they shape behaviour and raise costs. Yet history suggests they bend to strategic reality more readily than reality bends to them. War and human society—science, art, architecture, urban form, engineering—remain mutually constructive and destructive. The fortified disposition now re-emerging is one expression of that deeper reciprocity.

Mobile and offensive operations will continue to matter. The balance, however, is shifting. Those who persist in treating fortification as a historical curiosity risk being surprised by an operational environment that rewards those who prepare the ground, harden critical nodes, and dig deep.