This is the final part of a five part (I, II, III, IV) series covering some of the basics of fortification, all the way from ancient city walls to modern anti-access/area denial systems. Last week, we looked at the changes which gunpowder-based weaponry enforced on fortification design in Europe, leading to the emergence of the trace italienne.
This week, we’re going to look at the impact first of industrialized firepower (and then briefly at the end, modern precision weaponry) has on the design and purpose of fortifications. In practice, the relative stability of design in trace italienne forts between 1550 and 1750 reflected the fact that while cannon themselves represented an enormous change in warfare and continued to develop over the period, the challenge that cannon posed remained fairly constant. By contrast, the emergence of exploding shells, high explosives and the industrial power necessary to produce these in staggeringly large quantities posed a radically different, new set of challenges which, as we’ll see, fortress design struggled to adapt to. Because the changes in this period (c. 1870 to present) are very rapid this essay is going to be even more of a high-elevation overview, but I hope this will at least give a general sense of the shape of things.
As always, if you like what you are reading here, please share it; if you really like it, you can support me on Patreon. And if you want updates whenever a new post appears, you can click below for email updates or follow me on twitter (@BretDevereaux) for updates as to new posts as well as my occasional ancient history, foreign policy or military history musings.
Despite this tremendous increase in available firepower, many of the same strategic concerns motivated fortress construction in the decades prior to the First World War. The experience of the Franco-Prussian War had demonstrated to everyone that quick offensives supported by railroads could potentially overrun even relatively large countries (like France!) faster than those countries could actually mobilize their armies. Consequently, countries that worried they’d be slow to mobilize (Russia, Austria) or that they’d need to wait for other powers to come to their rescue (Belgium), looked to use permanent fortresses as a way of delaying offensives, slowing down that initial rush until they could have their own armies ready for a decisive field engagement (recall that no one, at this point, has envisaged entire fronts of a war settling down to trench warfare; the assumption is that there will be a decisive series of pitched battles, albeit on a massive scale). On the other hand, fortresses could also be intended to shape offensives, closing off certain lines of attack (the fancy military term we’ll come back to here is ‘canalize’); France engaged in this kind of fortress construction prior to both world wars.
The solution that European fortress designers came to were ‘fortress rings’ composed of ‘ring forts,’ the latter term a bit confusing because a ‘ring fort’ or ‘ring castle’ was an older style of keepless-castle during the Middle Ages. Here, as far as I can tell, the forts are termed ‘ring forts’ not because of their shape, but because they are arranged in a mutually supporting ring around a key defensive point. That point was typically a city, but what the forts are defending is no longer the population or administrative center, but the infrastructure center, since cities tended to be where roads and railroads came together which made cities ideal blocking positions to deny the enemy access to that transportation system (which modern armies increasingly relied on to move with any speed and to manage their logistics). That in turn matters because armies are no longer drawing most of their supplies from local foraging operations, but rather are now primarily being supplied by railway lines reaching back to depots and factories in the home country, in part because by this point ammunition requirements (particularly artillery) by mass and volume were now beginning to eclipse demand for things like food which might be acquired locally. Artillery shells, alas, are not regularly grown by farmers out in the countryside. Thus, prohibiting an enemy army the ability to use the railroad and roadway junctions within a major population center could dramatically limit the ability of that army to move beyond the city. In practice then, this is an effort to achieve the same kind of operational blocking effect that castles with cavalry detachments did.
We can take the Fortified Position of Liège (PFL or position fortifiée de Liège) as a good example of the system. Built beginning at the tail end of the 1880s, the fortress ring was made up initially of 12 forts of varying sizes, arranged in a very rough circle with a radius of around 5 miles, centered on Liège. The dramatic increase in the range and power of artillery meant that, despite these forts being widely spaced, they could support each other with heavy artillery fire and ideally also use their artillery to support a screen of infantry which, in a war, would be dug in in the roughly 1-3 mile wide intervals between the forts. In theory then, an attacker would be unable to isolate the forts from each other; pushing through ought to require neutralizing the forts one by one (though it doesn’t, in the event).
The design of the forts themselves were also responding to increasing firepower. The development of stable high explosives in the 1880s had triggered what became known as the ‘torpedo-shell crisis’ – torpedo-shell being the term at the time for artillery shells with a modern pointed cylinder shape and a high-explosive detonating charge inside. Such torpedo-shells (again, today we’d just call them ‘shells’) could punch into the earth under a fort and then explode with tremendous force, heaving the earth (and the poor defenders) upward. Such shells could easily obliterate even earthwork backed walls (like the old trace Italianne walls) or the above-ground brick casemates of older polygonal forts and they could do so from tremendous range, because unlike older siege guns, the striking power was in the explosive charge of the shell, not its raw velocity, meaning that it was no longer necessary to situate a cannon close to a fort for a direct-fire bombardment to breach the walls. This new danger forced the redesign of many older forts (for instance the Séré de Rivières system in France of which the fortress-system at Verdun was a part) and many of the fortresses of WWI were in the expensive process of modernization when the war started. The PFL, however, was begun relatively late (1888) and took these problems into consideration.
First, the forts, rather than being elevated above the surrounding terrain, were essentially submerged, built entirely in concrete below the level of the glacis so that enemy artillery couldn’t fire directly at the fort itself. The core armament of each fort, the heavy artillery, would be carried in rotating thick steel turrets (much like the guns of a contemporary pre-dreadnought), made to be nearly flush with the ground (to make it very difficult to land a direct hit on them with direct-fire artillery); ammunition was stored deep underground beneath these turrets and brought up, again, in much the same was as the main armament of a pre-dreadnought. The guns in question were quite large, with the larger forts having two 15cm guns, four 12cm guns, and two 21cm guns split between five turrets. To defend the heavy guns from infantry assault, each fort featured both smaller artillery (5.7cm) for direct-fire anti-infantry support. As with the older polygonal forts, the (dry) moat (here technically called the ‘gorges’) provided infantry fighting positions using the glacis as the edge of a trench, while at the same time the fort’s structures set down within the moat had firing holes and so could function more or less like caponiers, though as the firepower available to the defenders increased, it seems clear that concern about close-in-fighting in the gorge was lessened. Attempting to storm one of these forts on foot without first demolishing it through artillery must have seemed a mad gesture (and indeed, Russian efforts to storm somewhat older forts at Przemyśl this way failed miserably).
Moving to other ring fortress systems, where was substantial variation in the design of individual forts, often being shaped by the terrain and also because of the rapid pace with which they had to be redesigned and modernized to resist ever more powerful artillery. The Austrian fortress system ringing Przemyśl was less modern than that at Liège, but similarly based on the idea of mutually supporting artillery positions set in a ring, with the forts once again constructed as galleries dug down into the faces of the gorges, although much of Przemyśl’s older construction was in brick rather than concrete. The forts of the French defense system at Verdun were, in their 1914 form, closer to the design at Liège, although the French forts had been covered over with substantial amounts of reinforced concrete to fortify them against those torpedo-shells.
These fortresses were impressive, they were often center-pieces in the defense strategy of their countries and they mostly failed. The forts at Liège (and similar positions at Namur and Antwerp) had been designed to withstand 21cm artillery with the firepower of 1886; the German Army, knowing this, had specially designed 42cm ‘Gamma Guns‘ (and the famous 42cm howitzer, ‘Big Bertha‘) with the specific aim of being able to breach the fortresses of the day. At Liège , the other problem was that a barrage, even one that might not destroy the fort, rapidly rendered the fort spaces unlivable due to ventilation, water and sanitation issues caused by bombardment. This problem was intensified by design issues, namely that the living spaces in the fort had been pushed into the ‘counterscarp’ (the outer edge of the gorge wall), which wasn’t exactly the most pleasant or safest place to be when the fort was being shelled.
In the end, while the Belgian fortifications did buy some time for Belgian, British and French mobilization, it was far less than the month that had been planned (though far longer than the two days that the often unrealistic German timetables of the Schlieffen Plan had allocated). Invested on the 5th of August, the German army had punched through the thin defenses connecting the forts and taken Liège on the 7th and were then able to isolate and reduce the forts one by one, with the last surrendering on the 16th. The subsequent Siege of Namur went even more smoothly for the Germans, with the fortress system falling in just five days and the garrison of some 35,000 defenders being neutralized at the cost of only 900 German casualties.
Ironically, the collapse of the Liège forts convinced French high command that these fixed fortifications were unlikely to be held against determined German assault, leading to the progressive downgrading of the Verdun fortifications (their artillery was mostly removed for use amidst the trenches), despite the fact that the modernization efforts on the Verdun forts actually did render them relatively more resilient against artillery barrage, something the French would themselves find out when trying to retake forts Douaumont and Vaux after both were captured by the Germans in the opening of the Battle of Verdun. Fort Douaumont was captured by a small German reconnaissance party just four days into the battle, its defenders having been forced deep into the fort by the barrage and being caught unawares. Fort Vaux stood off the Germans better, holding out until June, but its defense was seriously hampered by the removal early in 1915 of nearly all of its artillery, leaving only a pair of 75mm guns in the turret. The French would retake both forts only after stunningly heavily bombardment; when defended, the forts seemed to work just fine.
In the more open environment of operations in the East, the other limitation with these fortresses became apparent: blockade. The fortress system at Przemyśl was nowhere near as modern as those at Liège or Verdun, but then the Russian army which laid siege to it in 1914 lacked the heavy fortress-busting artillery of the Germans. Efforts by the Russian Army to actually assault the forts or blast them initially failed. A first effort to take the city started in September, 1914 failed when the fortress was relieved by advancing Austrian armies on the 9th of October; but by the end of the month the momentum shifted and the Russians were back. Efforts to relieve the fortress by attacking through the Carpathians in winter – a series of catastrophic military mistakes by Conrad von Hötzendorf so bad that they nearly match the blunders of Luigi Cadorna – actually inflicted more losses on the Austrians (around seven times more losses) than were in the fortress itself, which fell in March, with more losses (including the surrender of the surviving 117,000 defenders) on the defender’s side than the attackers (not counting the Carpathian offensives), despite the defenders, in theory, having a fortress. Nevertheless, the Russians took severe losses in ill-conceived efforts to storm the forts with insufficient artillery before simply settling down for a siege and waiting the defenders out.
The Russians fared much worse in trying to hold fortresses against the Germans. Kicking off the Gorlice–Tarnów offensive on May 2, 1915, the Germans reached Przemyśl – now in Russian hands – on May 16, and captured the city on June 5. As Russian armies fell back in what became known as the ‘Great Retreat,’ fortresses which ought to have been able to buy valuable time instead became expensive traps. The Russian fortress at Kaunas had a garrison of 90,000 soldiers, while a large part of the Russian First Army, another 90,000 men, threw itself into the Novogeorgievsk fortress. The Germans took both handily; Kaunas was besieged on August 8th and the complex had fallen by the 19th while Novogeorgievsk was invested on the 10th and fell on the 20th. The loss of both, in particular complete with their shells and guns (2,900 artillery pieces and well over a million shells between them) was a disaster. The difference here was more ample German artillery – heavy artillery in particular – which made battering the defenses down practical.
But of course that statement was a damning one: a fortress which is only useful against enemies with substandard artillery and crippling shell shortages isn’t much of a fortress at all. In practice, while forts could resist assault for a time (as at Vaux or Przemyśl), in the face of the firepower now available to the attacker and vast industrial armies which could easily detach armies of tens or even low hundreds of thousands of men to bottle up and besiege a city without even pausing an offensive, the sort of pre-built static defenses that had defined European warfare since the initial development of the trace italienne were now quite clearly on the wane. But of course this is World War I, so that doesn’t mean fortifications are on the wane.
We’ve already discussed the trench stalemate that emerged on the Western and Italian fronts in WWI and to a lesser extent in many of theaters as well, so we don’t need to belabor the point here. What I want to note here are some of the ways that the trench networks that emerged by the end of the war reflected responses to the problem of heavy artillery, penetrating high explosive shells and firepower generally which had largely doomed the great ring fortresses of the pre-war era.
In particular, the systems of trench fortifications eventually embraced defense that was in depth and elastic. We’ve seen defense-in-depth as a concept quite a bit here, but again the fundamental notion of defense in depth is that because of friction (in the Clausewitzian sense, drink!) an attacker’s attack is never better organized nor stronger than at the moment it steps off; by forcing the enemy to engage a series of obstacles and disruptions, the defender turns that friction into their ally as the attack slowly breaks down due to confusion, casualties and random chance. While the pre-war ring fortresses in theory, by having multiple forts, embraced a defense in depth, in practice neutralizing just a handful of the forts in the ring compromised the entire position (in the case of Liège, the Germans actually compromised the whole fortress system without taking any of the forts; they punched through to the city first and then reduced the forts individually afterwards). An attacker could reduce the forts one-by-one, with pauses in between, or reduce just a few and then push through and take the rest from the rear. Consequently, the attackers could prepare carefully (in some cases, prepare for years since the fort’s layouts and defenses were well known) and unleash their full, un-frictioned attack on each fort one by one. It wouldn’t be inaccurate to say that the ring fortresses defeated themselves in detail.
By contrast, trench systems were deployed in depth. German doctrine was the most clear and sophisticated here and it eventually called for a standard three-line system (see below) with forward fighting positions which were to be abandoned, a main battle position, and then a third line with reserves which would, once the enemy attack had run out, counter-attack. These could be supported in turn by strategic reserves rushed along rail and road lines to the battle. British and French positions weren’t always so deep or well-organized, but they too were generally set in multiple mutually supporting lines. The problem, as we’ve discussed, wasn’t taking the forward positions, which was typically accomplished, but reaching the second or third lines with enough momentum to carry out into the open country, which was almost never achieved.
Defense in depth was also a response to the massive increase in firepower, particularly artillery firepower. By holding the front line with only a light force and keeping most of the troops in reserves to the rear, the bulk of the defending force could be kept out of the enemy’s initial, devastating barrage, safely out of range of all but the largest enemy guns. In essence, the thin defense of the first line of trenches functioned as a screening force, shielding the fresh troops to the rear from enemy artillery, both by physically keeping the artillery back and out of range but also by helping to shield those rear areas from observation. The importance of observation swiftly becomes crucial when artillery are capable of devastating fires at over-the-horizon targets. While we often imagine artillery just indiscriminately blowing up everything in a large area, in practice artillery is only really effective if it can put shells on target and even a notionally dense front line is still mostly empty ground. Consequently, if you can deny enemy observation, you can dramatically reduce the effectiveness of their artillery (there’s also the related issue of shot adjustment; complex atmospheric conditions means that adjusting after each shot is more accurate than firing blind from range tables and formulas). As we’ll see, that focus on not being observed as a key component of defending against modern firepower is only going to become more prominent.
Finally, this is an elastic defense.1 Elastic defense was hardly a new concept; Roman frontier defenses, as we noted, were elastic on an operational level and the many layers of large, complex trace italienne forts were meant to be elastic in a tactical sense. Elastic defense is a much more active model of defense, wherein the defender does not merely receive and parry attacks but makes their own spoiling attacks and counter-attacks, both to regain defensive positions ceded to the enemy but also to disrupt enemy plans.
While in theory the pre-war ring fortresses had positions from which the defenders might launch sallies out, in practice those routes were generally cleared of obstructions and so would be swept by enemy fire too; the crushing power of modern industrial firepower made such direct sallies into no man’s land ill-advised. Moreover, secondary positions from which the defenders might launch assaults to dynamically retake defensive positions were typically missing; at best one might hope the garrison of the next fort down the line might try something, but in the event that fort, being fully exposed itself on the front line, would be in no position to lend aid to its neighbors, much less stage its own fortress assault. Instead, the defense of a ring fortress was typically rigid; forts once taken were not generally retaken; the clear exceptions at Verdun having more to do with those forts having been integrated into a trench network than being features of the fort’s original design.2
By contrast, the trenchlines of the Western front were always intended as both defensive positions and offensive stepping off points such that retaining the ability to ‘go mobile’ was crucial. Artillery might be dug in, with added overhead cover and earthwork berms to protect against enemy shelling, but the guns in question were still intended to be mobile and able to be redeployed from one sector to another (and indeed, just about every assault plan assumed that the attacker’s guns would eventually need to be moved forward to cover the advance, although actually doing this over the morass of the trench stalemate was often nearly impossible), while infantry was expected (by the end of the war) to dynamically shift from defense to counter-attacks as the opportunities arose.
As the war wore on, the importance of counter-attacks forced alterations in trench designs, leading to the main fighting positions being moved into the second trench line such that the intended response to an attack was to attrition the enemy before allowing them to take the first trench line, then stopping their attack at the second line – the main battle position – before reserves in a third set of positions would move up through pre-cleared communications trenches to launch a counter-attack to retake the forward positions. Since the enemy was likely to still be trying to push forward, those counter-attacks were also supposed to be flexible, shifting from defense to counter-attack as conditions changed (see the diagram above).
Thus it is a strange irony that the war which saw the beginning of the decline of fortress cities (not withstanding France’s effort with the Maginot line) became a war known for the strength of its fixed defenses, which remained virtually unmoved after four years of carnage.
Field Fortifications in the Modern System
As we’ve discussed at some length, the last years of World War I and much of the military developments of the interwar years were directed towards avoiding the brutal trench stalemate; the end result of these developments was what Stephen Biddle describes as the ‘modern system.’ Incorporating developments in mobile warfare (that is tanks, combined with motorized or mechanized infantry), air power (particularly close air support) and infiltration infantry tactics (also known as ‘Hutier‘ tactics), the modern system aims to use mobility, cover and concealment to avoid the overwhelming firepower of modern industrialized war in order to avoid the trench stalemate and retain a war of maneuver. Not every modern country’s military methods embrace the modern system, because it is expensive and difficult, but the modern system finds expression in a range of industrial military doctrines, particularly those of the most capable militaries.
This continued development of firepower has led quite a lot of observers to thus assume that the offensive is now all-dominant and that defenses, including fortifications, are a relic of the past (an idea often expressed through Patton’s misquoted contempt of the Maginot Line, presenting it as the both the last great fortification and also the marker of fortification’s failure). But as ADP 3-90 Offense and Defense (2019)3 notes, “While the offense is more decisive, the defense is usually stronger,” (4-1) a statement that is a near direct quote of Clausewitz, “the defensive form of warfare is intrinsically stronger than the offensive…yet has a negative object” (6.1.2, p. 358 in the Howard and Paret translation; drink!).
Playing the Clausewitz drinking game with US Army field manuals would be, by the way, a pretty good way to get really hammered really fast, but you’d need to know your Clausewitz, because the borrowings are typically unmarked.
That said, the defense envisaged in ADP 3-90 is not a static defense. Instead, the manual notes “a defending force does not wait passively to be attacked” but “seeks ways of attriting and weakening enemy forces.” Maneuver and concentration even on the defense is stressed, as is a willingness to give ground in order to either buy time for maneuver or wear down enemy forces. In short then the defensive stance that is envisaged here is an elastic defense, but one that stresses even more flexibility and speed, which fits with the modren system, especially since the mobility demanded here is in part to retain concealment since once a force has been engaged, its position is known and so it must move in order to re-conceal itself. And of course the goal of the defensive, here as in Clausewitz (drink!) is always to get back on the offensive.
So what place do fortifications have in this vision? Quite a bit, actually, but the mobility and flexibility of the doctrine lead to an emphasis on far more temporary defensive positions, held for a given purpose and given up as soon as that purpose has passed. The ATP 3-21.8 Infantry Platoon and Squad (2016) field manual lays out the basics for small units. “Preparations” – which include both utilizing natural obstacles and enhancing those obstacles to aid in a defense “end only when the defenders retrograde or begin to fight” and that “preparations in-depth continues, even as the close fight behinds.” Strongpoints designed to block or canalize enemies are important; the manual notes such a position typically takes roughly a day for an engineer detachment of equivalent size to construct (with ATP 3-37.34/MCWP 3-17.6 Survivability Operations laying out the ways that existing structures and terrain can be reinforced). The emphasis in positioning, as with late WWI-trenches is in channeling (‘canalizing’) enemy attacks into areas with overlapping fields of fire or exposed to indirect fires (that is, artillery). But the modern system emphasis on maneuver comes out clearly as well. The manual twice lays out expected phases or sequencing of the steps for a defensive action (in 3-47 and 3-70), in both cases ending in swift maneuver and counterattack. You let them hit you, defeat their advance, maneuver against them, and then strike before they can attack again, getting back to mobile warfare as quickly as possible.
Fighting positions (discussed in ATP 3-37.34/MCWP 3-17.6 Survivability Operations, chapter 4) are fortified, but they aren’t fortresses. At the beginning are ‘hasty positions’ – using what the local terrain and structure offers, combined with basic dig in to protect against direct fire (though the field manual notes such quickly dug positions offer “no overhead protection” – the concern here being the modern version of those bursting shrapnel shells). “Deliberate positions” follow with more time (once again “continued improvements are made…during the period of occupation,” so the longer you are there, the more works you put up), with an emphasis on concealment (using camouflage, or existing structures), overhead cover, and opening protected routes connecting positions. Some of the positions described imply several days of preparation, but not too much more because of course the longer a fighting position is occupied, the more likely it is to be spotted and hit with overwhelming enemy firepower. The doctrine here largely does not address situations where formations are in contact with the enemy for months at a time because in this doctrine that isn’t supposed to happen (whereas in WWI that was the exact condition in which the trenchlines emerged).
I should note that all of these manuals also have considerations the impact of tactical nuclear weapons on these defenses Defense in depth is recommended in ADP 3-90 expressly when there is a threat of weapons of mass destruction (much the way it was used to limit exposure to massive conventional artillery barrages in WWI) and ATP 3-37.34 assesses fighting positions based on the protection they provide, inter alia, in the event that nuclear weapons are used. In essence the potential of the use of nuclear weapons takes the firepower/concentration problem and dials it to 11. On the one hand, the defender, like an attacker, wants to be concentrated so as to deliver maximum force at the point of decision (in this case, at the point where the enemy attacks), but also wants to be dispersed so as to avoid losing the entire unit to the devastating impact of modern firepower. Concealment and mobility are presented as the solutions: concentrations can be maintained only for the decisive moment and then an immediate return to maneuver to deny as much as possible that concentrated, target-rich position for the enemy to deploy their massive firepower, be it conventional or nuclear, on.
Meanwhile, US Army doctrine envisages more extensive fortifications for permanent base camps and forward operating bases (FOBs); these aren’t (in theory) frontline fighting positions, but could come under attack (especially indirect fires or guerilla attack) and so have to be fortified against that kind of attack. The basics are covered in ATP 3-37.10/MCRP 3-40D.13 Base Camps (2017), which in turn frequently refers back to ATP 3-37.34 for the particulars of fortification design. Such bases are to be fortified, typically with a single-layer perimeter (that is, a a non-concentric defense, but that defense typically has multiple elements) in a rectangular shape supplemented with internal barriers. The outer barrier generally consists of a combination of barbed wire, a physical barrier (that is, a wall) and earthworks (typically ditches, but also berms). The wall can be made out of many kinds of materials, but as far as I can tell, the HESCO barrier, made up of a wire-mesh container which can be rapidly filled with earth and set in rows and stacked to create a barrier, is the most common. Interestingly, this is an old technology come back: the venerable early modern gabion in a modern, industrialized form. As with the gabion, the advantage of HESCO barriers is that it ‘catches’ projectiles rather than either shattering or causing the projectiles to ricochet. The field manuals also note the use of similar barriers within the base to catch shrapnel from enemy artillery or mortar fires (see the image below on how you can ‘compartmentalize’ shrapnel this way). So a complete perimeter might consist of a ditch, then a line of HESCO barriers, topped with wire to prevent scaling, with the interior base then subdivided by more earthworks or HESCO barriers to reduce the impact of shrapnel.
That basic perimeter is then reinforced with fighting and observation positions. Towers sit inside the perimeter (often on the corners) but flush with the outer wall to maximize observation while still benefiting from the protection of the perimeter wall; they can double as (and also be supplemented by) dedicated fighting positions on the perimeter. The gate – or ‘entry control point’ (ECP) is also reinforced and guarded – access points are to be designed so that vehicles can’t approach the perimeter in a direct, perpendicular line, but have to turn into the base, using “sharp turns and serpentine layouts” to mitigate high speed vehicle attacks and generally frustrate enemy efforts to access the gate.
And at this point I hope this combination of features is familiar: a rectangular shaped outer earthwork supplemented with an anti-climb device on the top, ditches to frustrate approach and observation towers flush with the wall on the inside of the corners, with a gate reinforced with a serpentine layout…because that description could also describe a Roman marching camp. And for much the same reasons, because while the types of weapons have changed, the basic threats the design aims to avoid are the same: the goal is to maximize observation while slowing down enemy access to that the army inside the camp has time to mount an effective pitched-battle response. Both designs also show a significant concern with the possibility of enemies seeking to fire into the camp from a distance; both often place an interval between outer barriers and the internal structures to mitigate the risk of weapons thrown into the camp (be they grenades or javelins). I don’t know that US Army planners aimed to reconstruct a modernized version of the Roman playing-card fort, but the concerns of force protection for an army on the move (and then an army settling down to try to establish durable control and engage in COIN) remain largely the same and so lead to similar designs.
One wonders how central these sorts of fortifications will be in the history of the American limes; will archaeologists marvel at the striking similarities in design between American FOBs located many hundreds of miles from each other? But in any case, modern warfare has not removed the need or value of field fortifications, although within the modern system, the emphasis on mobility remains very strong. But is there a future for fortifications beyond the modern system?
A2/AD and the Future of Defense
Predictions are hard, especially about the future and not generally the province of historians. Nevertheless, defensive technology has not stood still and it could be argued that several new technologies may shift the balance back from the apparent supremacy of the offense and thus bring back a form of fortifications, albeit of a kind that won’t look very much like the castles and concrete pillboxes of old. Still, such technologies aim to accomplish the same goals, raising the cost of the offensive unacceptably high and channeling attacks that do occur and so we might well term their use a form of fortification.
Now defense and fortifications are not necessarily identical. Nuclear deterrence is, after all, a form of defense mostly4 without fortifications: a defense through almost pure, offensive retaliatory power. That said, missile defense aimed at interdicting ICBMs opens the possibility of fortifying an area against nuclear strike using ICBMs. My sense is that public perception of missile defense begins with SDI (the ‘Strategic Defense Initiative’ also known as ‘star wars’) in the 1980s, but the idea of a fortification system designed to prevent a nuclear strike goes back at least as far as the Nike-Hercules missile, developed in 1958 and the later Nike-Zeus, both of which could in theory intercept ballistic missiles (with their own nuclear payload), though never with sufficient reliability. As a personal aside, when I was a kid, very near my elementary school was ‘Nike Park’ so named because it was one of the 13 Nike Missile sites positioned around Washington DC to provide the capital with a defense system; decommissioned in 1962 it was turned into a park with the old radar dome being left as a cool echo chamber.
While SDI didn’t fulfill expectations, the 90s and aughts saw leaps to where missile defense systems now appear to be feasible and effective and a number of different countries have developed them. The United States has the Ground-Based Midcourse Defense, administered by the Missile Defense Agency, which manages a $9.18bn budget. While that reflects a significant investment in absolute dollar terms, compared to military spending more broadly (both in the USA and globally) it reflects a belief that the probability of a nuclear exchange is low. Consider that the annual cost of the Missile Defense Agency is just 0.04% of the United States’ GDP; for comparison, the initial construction of the Liège forts (the PFL) alone cost more as a percentage of GDP (around 0.065% if I have done my currency conversions right; alas I have the GDP of Belgium in 1880 in 1880Int$ but the cost of the forts in Belgian francs) to build and Belgium of course built equally expensive fortress rings around Namur and Antwerp and then had to supply and man these fortifications.5 The problem, at least for now, with ballistic missile defense is that the two leading nuclear arsenals, those of the United States and the Russian Federation are so large that they could simply overwhelm current defense systems, but as a defense against smaller nuclear powers, the possibility of fortifying a country with anti-ICBM systems now appears, if not realized, at least within the realm of possibility, especially if a rising sense of nuclear threat caused a radical increase in the level of investment in such systems.
Meanwhile, conventional warfare and defense hasn’t gone away. While the tradition fortresses of previous eras have largely been discarded in the post-WW2 era, the so-called Revolution in Military Affairs has made stand-off precision weaponry a possibility, extending the range from a border or coastline that a defender can seek to exclude enemy forces with the threat of precision strikes delivered by cruise missile or drone. These sorts of systems fit under the headline of A2/AD (anti-access, area denial) weapons because their goal is to prevent an enemy from moving through or operating in a specific area (though the term can also mean older forms of area denial weapons like land mines or even caltrops, the meaning of A2/AD in policy circles these days almost exclusively refers to this very modern meaning of the term). A2/AD systems allow a country which might not actually be in a position to directly contest a battlespace (because its army or navy is not confident of direct victory in that space) to nevertheless deny it to an enemy by ensuring that the area is covered by enough precision systems that the attacker would take unacceptable losses attempting to operate in the space, which can in turn mean denying the enemy the ability to actually deliver forces to one’s own country as a form of defense. The discussion of these sorts of systems has been most prominent in terms of China’s sea-directed A2/AD system, but such stand-off A2/AD systems look to be a durable component of national defense, at least for countries wealthy enough to afford large numbers of the expensive and sophisticated systems.
The emergence of the related ‘porcupine’ strategy, particularly in the context of the country6 of Taiwan, which aims to combine both the potential of A2/AD systems with the potential intractability of a motivated insurgency to raise the cost of attacking even a small (but wealthy, because some of these systems aren’t cheap!) country to unacceptable high levels, presents another avenue for future fortification. The core of a porcupine strategy, as laid out so far, is the use of both anti-land cruise missiles, anti-air and anti-ship missiles based in expendable and concealable platforms, either tractor trailers or small missile boats (which might, if small enough, be able to hide as fishing boats), combined with a final ground defense oriented more around irregular capabilities which would be difficult for an attacker to target with their own precision systems (that is, infantry with man-portable weapons, not expensive tanks and jets). Essentially, deter an attack by promising the enemy they’ll have to wade through lots of long-range precision strikes merely for the honor of getting to wage a long and difficult counter-insurgency campaign at the other end.
I think it is not at all unreasonable to see developments like contemporary A2/AD and the related ‘porcupine’ strategies as a return to fortifications of a type. Just as the industrial firepower of the late 1800s forced the single forts or fortress systems of the previous era to be broken up into the ring fortresses, so continued military development has demanded that defensive capabilities continue to be broken up, spread out and most importantly concealed in order to be effective. But a country which invests heavily in mobile, concealable A2/AD assets in order to deter attack is engaging in fortification of a kind, building physical infrastructure for the purpose of deterring or stopping an enemy attack.
So are “fixed fortifications…a monument to the stupidity of man?” Well, once again, because I hate misquotes, I feel the need to note that what George S. Patton actually said was, “this is a first class case of man’s monument to stupidity” about the Maginot line. But more broadly, the answer here is clearly ‘no.’ Fortifications made sense historically. The current popular conception that ‘fixed’ fortifications are useless is a consequence of a shift towards the offensive, particularly during the Second World War, but the balance between the offensive and the defensive is ever-shifting and never stable and may even now be shifting back towards the defensive due to the relative vulnerability of conventional offensive military forces to modern precision weapons (and the relative difficulty of using those same systems effectively against local insurgent forces).
In the end, the continued relevance of fortifications comes from the continued strategic imperatives: for weaker powers to find ways to deter attackers or at least shift the balance of strength so that a weaker power might win and for stronger powers to force a conflict to occur in favorable conditions. So long as there are polities operating under those conditions, they are going to try to find ways to fulfill those objectives. Not all of those methods will be fortifications, mind you – we haven’t much discussed insurgency here, but it can be seen as a non-fortification defensive strategy when adopted at the state-level as a planned fallback for when conventional arms fail – but some of them will be. The methods change, but the enduring strategic imperative means that fortifications, in one form or another, are also likely to endure – though we should also be aware that just as the forms of fortifications have changed radically in the past, they are likely to change radically in the future.
Happy New Year everyone!
- Elastic defense and defense-in-depth are often treated as synonymous, but I am actually trying to build a distinction here between a defense that is designed to fail in sequence and utilize enemy friction against them (defense in depth) and a defense that assumes quite a bit of active maneuver, envisaging both retreats and advances within an overall defensive framework. In this sense an elastic defense is a sub-type of a defense in depth which expects to transition flexibly between its various layers, rather than merely falling back to each in turn; under this rubric all elastic defense is in depth, but not all defense in depth is elastic. For instance a defense-in-depth of a castle that is non-elastic is one that expects to first defend at the outer wall, then an inner wall, and then the keep, and hopes that by the time the assault reaches the keep’s door, the attacker is too spent to continue. By contrast an elastic defense of the same castle might envisage sallies beyond the wall, or sorties to retake parts of the outer wall even after the enemy’s main assault has been launched.
- Some forts, particularly those in Belgium, were designed to be more vulnerable to assault from the rear, but this turned out to be a disastrous mistake as there was no ‘back-up’ system of defenses to prevent an attacker, having breached the ring, from then rolling up each fort in turn from its vulnerable rear, nor were they secondary positions designed to let the defender actually utilize this vulnerability, in stark contrast, for instance, to the hollow ravelins of trace italienne forts where the clear and obvious position from which to retake a ravelin was the bastion that sat behind it and could fire directly into its works. Alas at Liège the fort system had only half of this good idea which had perplexingly already been fully developed in the 16th century
- A United States Army field manual. I am going to lean quite a lot on US military field manuals here because a lot of US doctrine is declassified and available to the public, which gives a really clear view into how one of the world’s premier militaries functions and thinks about these things
- Of course individual nuclear installations – silos, command centers, and so on – are often heavily, even extravagantly fortified to render them resistant to nuclear attack, but that is merely to ensure the durability of the second strike retaliation. The actual target of a nuclear strike – cities, conventional military forces – cannot be sufficiently hardened to resist such an attack and typically no attempt is made to do so.
- Another comparison here might be the Manhattan Project, which is estimated to have cost around 0.8% of US GDP during its development.
- Yes, I said it.