On War, along with other memoirs and histories of its era, concerns itself in large part with fortresses. Large, partly self-sustaining fortified areas (food production was always impossible) continued to be important to military strategy even after cannon brought down the old castle walls.
Fortresses had several characteristics:
They controlled strategically important terrain, such as roads, mountain passes, and riverways. Typically, these features passed through the fortress.
They were large enough to house substantial bodies of troops, materiel, and supplies, well in excess of the permanent garrison.
They allowed armed forces inside to sally and return at will either en masse or in small groups.
They were sufficiently protected, both by fortification and armament, to require a concerted “main effort” to reduce. Successfully storming the fortress (in modern terms an “assault”, though “storm” is still used in German) required the most effort; an “investment” or siege to prevent sallies and eventually starve out the garrison a lesser effort. An “observation” or screen could be placed around a fortress with even less resources, although this would still allow entry and exit from the fortress more or less at will. In the 17th century, an entire field army might be required to reduce a fortress; as armies grew larger this became less the case, but fortresses remained an important feature while declining in importance compared to field fortifications through World War I.
What Happened?
There are two culprits for the decline of the fortress: mobility and firepower.
First, the increased mobility of both mechanized and air-dropped ground forces made it much more difficult for static fortifications to control terrain features. Motorized and aerial supply lines also make it much more difficult for a bypassed position to disrupt the communications of the bypassing unit.
However, the increased range, power, and precision of modern weapons clearly has the most to do with the decline of the fortress. Air-dropped munitions can easily destroy most fortifications, and modern munitions will hit whatever can be detected from outside the range of defensive armaments. The last identifiable fortresses — such as Malta — relied in the final assessment on the inaccuracy of aerial bombs to survive.
What would a Modern Fortress look like?
A modern fortress would have the characteristics described above, but I will add an important qualifier:
It should cost more to destroy than to build.
I won’t get too wrapped up in this, but: if our fortress takes a great deal of effort to create, but can easily be eliminated by a readily-available, inexpensive weapon or attack, it’s not what we’re looking for.
I’d also like to add something else in to make it more specific, based on the concept of “surfaces and gaps”. A surface is, basically, a strong point (not necessarily a physical location) and a gap is an exploitable weakness. A surface or gap does not need to always be such to everyone all the time. An air defense battery presents a “surface” to an aircraft (when it’s turned on!) but a gap to an infantry platoon.
A fortress should be “all surface” to an attacking force.
With this in mind, what are modern equivalents?
Airbases
Both the Donetsk War (Donetsk Airport) and the Syrian Civil War (Menagh, Al-Tabqa, Al-Duhur, Kuweires) have seen airports and air bases hold out against enemy attack long after surrounding territory. All of these sites had protective perimeter fortifications, and held out far longer than the surrounding territory in the face of enemy attack (in the Syrian case, for several years). Donetsk did not last as long, but was also not as extensively hardened.
Menagh Air Base — The defending SAA occupied the airbase, while attacking rebels operated in the town to the northeast.
Donetsk Airport in early 2015
One thing that all of these had in common, though, are the lack of an opposing air-to-surface threat. While the enemy was reasonably well equipped in both cases — including artillery and armor — they had no ability to attack these installations from the air. In Syria, the government was able to keep the defending forces resupplied via helicopter for several years until defeat or, in the case of Kuweires, relief.
Forward Operating Bases
Expeditionary Forward Operating Bases (FOBs) have many similarities to the airbases above. However, they are purpose-built to actively support ground tactical operations outside their perimeter, as the examples above are not. They do include both perimeter fortifications and point-defense systems to destroy incoming standoff munitions. American “air defense” at bases has mostly been focused on point defense against crude artillery. Additionally, they may deployed integrated air defense systems (IADS).
Camp Bastion in Afghanistan
Typically an IADS is conceived as protecting an area, but this needn’t be the case. The Russian military base in Syria at Khmeimim is clearly the protected objective of such a network. Unlike the typical American FOBs, the Russians clearly consider a sophisticated aerial attack a real possibility. In accordance with the “all surface” condition above, the defense system shouldn’t require any component outside of the perimeter defense, although this doesn’t preclude the positioning of air defense sensors outside the perimeter, nor the interlinking of the base with a larger network (as indeed is implied by the very term IADS).
Point defenses, however advanced, could almost certainly not prevent a hit from an ICBM or other suborbital attack, but anything short of that remains an open question. Currently, these air defense systems appear key to the creation of fortress-type installations.
I think it worth noting that any FOB worth its salt has at least limited facilities for air transport, and larger ones inevitably have full-service airstrips.
Tunnel Complexes
On organization that didn’t have the benefit of air superiority was the Viet Cong. Their most extensive tunnel complexes, I believe, meet the definition of a fortress given above. The most famous example is at Cu Chi:
The tunnels were by all accounts nowhere near as cozy as this illustration implies
This differs from a simple bunker in enabling stockpiling of supplies, providing assembly areas, etc. Being underground provides both protection and concealment. The camouflage aspect is most interesting, I think: if every everything that can be seen can be hit, and anything hit is destroyed, then you must not be seen.
Many of the tunnels — including the Cu Chi headquarters — were eventually destroyed. However, this was not done without an intense, concerted effort, and the tunnels operated for years on end.
The South China Sea Islands
The Chinese installations in the South China Sea (such as the one below in the Spratly Islands) may be the closest modern equivalent to a fortress. Like the airbases and FOBs above, they have clear fields of fire in all directions (being islands); this is a common feature except for the tunnel complexes. They’re also large enough to allow critical defense systems some room to move around on the island; only the runway itself is completely fixed. This need for some local dispersion is also common to the airbases above, and to some degree is even present in the tunnel complexes, which could dig out new tunnels or shift into new areas as needed.
Chinese installation in the Spratly Islands (Fiery Cross Reef). The runway is about two miles long.
Cities
Important towns and cities are often located on strategically significant terrain, and may constitute such terrain themselves. As a result, fortified towns and cities are nothing new. Unlike the previous examples, seamless perimeter defense appear no longer feasible. Like tunnel complexes, their defense generally allows for some limited penetration by an attacking force.
Cities are now much larger, relative to armed forces of any size, than in the historical past. This makes the defense or investment of a city a much more fluid affair than in the past, where city walls and a complete countervallation were standard features of warfare.
21st-century cities are a challenging military problem, but I believe they are better conceived as a complex sort of terrain rather than as a “fortress”, which is fundamentally a “point” position. An air-defense network could easily cover a large city, but the city itself strikes me as too permeable to enemy attack to constitute a fortress. However, the terrain of a city could be conceived as allowing for a “fortress”-like strongpoints, providing camouflage. Such strongpoints are ubiquitous in urban warfare.
The thought of a modern city truly converted into a true fortress is intriguing. The defender would have to construct a perimeter defense, and the garrison would almost certainly require the mobilization and arming of the city’s inhabitants in an organized manner. The coordination of the defense would be an incredible undertaking that would probably build on existing law-enforcement and governance structure. The mass-mobilization aspect runs against current trends towards smaller, professional regular forces; the garrison would almost certainly be outside of the “regular” armed forces structure. Vague gestures in this direction are sometimes background in science fiction, but nothing like it exists in the real world. An interesting concept.
Conclusions
The ability both to defend against aerial attack, and to enable the defender to conduct at least limited air operations, looks like a key capability for a modern fortress. Air defenses have been improving — the ability to intercept both low-flying cruise missiles and high-flying aircraft. Point defense systems might actually encourage air defense components to be placed more closely together (by reducing the threat of HARM weapons and the like), encouraging perimeter defenses. A look at the images above suggests that some level of dispersion inside the “fortress” is another key defensive capability, giving “garrisoned” artillery and air defense systems a space to move around against counterfire. Defensive dispersion and space for operating aircraft appear to be synergistic: you need a square mile or few to conceal the locations or your important defense systems, so you might as well have a runway in there.
The necessity for perimeter barriers and clear fields of direct fire (the needed range extended by modern weapons) remains. The Chinese installations in the South China Sea clearly has these concepts in mind, as do Russian-operated bases in Syria.