Fun unimportant thought: As tunnel density increases, there’s a phase shift in how warfare works (in the area with dense tunnels).
Consider e.g. a city that has a network of tunnels/catacombs/etc. underneath. Attackers can advance on the surface, and/or they can advance under the ground.
For tunnel networks of historically typical density, it’s better to advance on the surface. Why? Because (a) the networks are sparse enough that the defenders can easily man every chokepoint underground, and (b) not being able to use airplanes or long-ranged weaponry underground seems to advantage the defender more than the attacker (e.g. can’t use artillery to soften up defenders, can’t use tanks, can’t scout or bomb from the air). OTOH your attacking forces can get real close before they themselves can be shot at—but this doesn’t seem to be sufficient compensation.
Well, as the density of the network increases, eventually factor (a) reverses. Imagine a network so dense that in a typical 1km stretch of frontline, there are 100 separate tunnels passing beneath, such that you’d need at least 100 defensive chokepoints or else your line would have an exploitable hole. Not enough? Imagine that it’s 1000… The point is, at some point it becomes more difficult to defend underground than to defend on the surface. Beyond that point fighting would mostly happen underground and it would be… infantry combat, but in three dimensions? Lots of isolated squads of men exploring dark tunnel networks, occasionally clashing with each other, forming impromptu chokepoints and seeking to outflank?
Factor (b) might reverse as well. Perhaps it’s entangled with factor (a). In a superdense tunnel network, I could see it being the case that the advantage of the attacker (being able to advance unseen, being able to advance without being caught in fields of fire by machineguns and artillery, until they are literally in the same hallway as the enemy) outweigh the advantage of the defender (good cover everywhere, defensive positions can’t be spotted in advance and pounded by artillery and bombs) Whereas in a light tunnel network, there are only a few ways to approach so the defender just has to fortify them and wait.
I think a crucial factor that is missing from your analysis is the difficulties for the attacker wanting to maneuver within the tunnel system.
In the Vietnam war and the ongoing Israel-Hamas war, the attacking forces appear to favor destroying the tunnels rather than exploiting them to maneuver. [1]
1. The layout of the tunnels is at least partially unknown to the attackers, which mitigates their ability to outflank the defenders. Yes, there may be paths that will allow the attacker to advance safely, but it may be difficult or impossible to reliably distinguish what this route is.
2. While maps of the tunnels could be produced through modern subsurface mapping, the defenders still must content with area denial devices (e.g. land mines, IEDs or booby traps). The confined nature of the tunnel system forces makes traps substantially more efficient.
3. The previous two considerations impose a substantial psychological burden on attacking advancing through the tunnels, even if they don’t encounter any resistance.
4. (Speculative)
Imagine a network so dense that in a typical 1km stretch of frontline, there are 100 separate tunnels passing beneath, such that you’d need at least 100 defensive chokepoints or else your line would have an exploitable hole.
The density and layout of the tunnels does not need to be constant throughout the network. The system of tunnels in regions the defender doesn’t expect to hold may have hundreds of entrances and intersections, being impossible for either side to defend effectively. But travel deeper into the defenders territory requires passing through only a limited number of well defended passageways. This favors the defenders using the peripheral, dense section of tunnel to employ hit-and-run tactics, rather than attempting to defend every passageways.
(My knowledge of subterranean warfare is based entirely on recreational reading.)
As a counterargument, the destruction of tunnels may be primarily due to the attacking force not intending on holding the territory permanently, and so there is little reason to preserve defensive structures.
Thanks! I don’t think the arguments you make undermine my core points. Point by point reply:
--Vietnam, Hamas, etc. have dense tunnel networks but not anywhere near dense enough. My theory predicts that there will be a phase shift at some point where it is easier to attack underground than aboveground. Clearly, it is not easier for Israel or the USA to attack underground than aboveground! And this is for several reasons, but one of them is that the networks aren’t dense enough—Hamas has many tunnels but there is still more attack surface on land than underground. --Yes, layout of tunnels is unknown to attackers. This is the thing I was referencing when I said you can’t scout from the air. --Again, with land mines and other such traps, as tunnel density increases eventually you will need more mines to defend underground than you would need to defend aboveground!!! At this point the phase shift occurs and attackers will prefer to attack underground, mines be damned—because the mines will actually be sparser / rarer underground! --Psychological burden is downstream of the already-discussed factors so if the above factors favor attacking underground, so will the psychological factors. --Yes, if the density of the network is not approximately constant, such that e.g. there is a ‘belt of low density’ around the city, then obviously that belt is a good place to set up defenses. This is fighting my hypothetical rather than disagreeing with it though; you are saying basically ‘yeah but what if it’s not dense in some places, then those places would be hard to attack.’ Yes. My point simply was that in place with sufficiently dense tunnel networks, underground attacks would be easier than overground attacks.
With modern mobility (air, drones, fast armor, etc.), it’s not clear that “your line having an exploitable hole” is preventable in cities and built-up areas, even without significant tunneling. For “normal” tunnels (such that there could be one every 10 meters in a kilometer as your example), as distinct from “big” tunnels like multilane highways and underground plazas, it doesn’t take much to fortify or seal off an exit, once known, and while it’s not clear which side will decide it’s more danger than help, one of them will seal it off (or collapse it or whatever). Surprise ad-hoc tunnels are problematic, but technology is still pretty limited in making and disguising them.
Note that really effective small, unmechanized and unsupported surprise attacks are only really effective with a supply of suicide-ready expert soldiers. This is, logically, a rare combination.
That said, I don’t much have a handle on modern state-level or guerilla warfare doctrines. So I’d be happy to learn that there are reasons this is more important than it feels to me.
Edit to add: I get the sense that MUCH of modern warfare planning is about cost/benefit. When is it cheaper to just fill the tunnels (or just some of them—once it’s known that it’s common, there won’t be many volunteers to attack that way) with nerve gas or just blow them up, than to defend against them or use them yourselves?
Why would the defenders allow the tunnels to exist? Demolishing tunnels isnt expensive, if attackers prefer to attack through tunnels there likely isn’t enough incentive for defenders to not demolish tunnels
The expensiveness of demolishing tunnels scales with the density of the tunnel network. (Unless the blast effects of underground explosives are generally stronger than I expect; I haven’t done calculations). For sufficiently dense tunnel networks, demolishing enough of them would actually be quite expensive. E.g. if there are 1000 tunnels that you need to demolish per 1km of frontline, the quantity of explosive needed to do that would probably be greater than the quantity you’d need to make a gigantic minefield on the surface. (Minefields can be penetrated… but also, demolished tunnels can be re-dug.)
I think the thing that you’re not considering is that when tunnels are more prevalent and more densely packed, the incentives to use the defensive strategy of “dig a tunnel, then set off a very big bomb in it that collapses many tunnels” gets far higher. It wouldn’t always be infantry combat, it would often be a subterranean equivalent of indirect fires.
Thanks, I hadn’t considered that. So as per my argument, there’s some threshold of density above which it’s easier to attack underground; as per your argument, there’s some threshold of density where ‘indirect fires’ of large tunnel-destroying bombs become practical. Unclear which threshold comes first, but I’d guess it’s the first.
Fun unimportant thought: As tunnel density increases, there’s a phase shift in how warfare works (in the area with dense tunnels).
Consider e.g. a city that has a network of tunnels/catacombs/etc. underneath. Attackers can advance on the surface, and/or they can advance under the ground.
For tunnel networks of historically typical density, it’s better to advance on the surface. Why? Because (a) the networks are sparse enough that the defenders can easily man every chokepoint underground, and (b) not being able to use airplanes or long-ranged weaponry underground seems to advantage the defender more than the attacker (e.g. can’t use artillery to soften up defenders, can’t use tanks, can’t scout or bomb from the air). OTOH your attacking forces can get real close before they themselves can be shot at—but this doesn’t seem to be sufficient compensation.
Well, as the density of the network increases, eventually factor (a) reverses. Imagine a network so dense that in a typical 1km stretch of frontline, there are 100 separate tunnels passing beneath, such that you’d need at least 100 defensive chokepoints or else your line would have an exploitable hole. Not enough? Imagine that it’s 1000… The point is, at some point it becomes more difficult to defend underground than to defend on the surface. Beyond that point fighting would mostly happen underground and it would be… infantry combat, but in three dimensions? Lots of isolated squads of men exploring dark tunnel networks, occasionally clashing with each other, forming impromptu chokepoints and seeking to outflank?
Factor (b) might reverse as well. Perhaps it’s entangled with factor (a). In a superdense tunnel network, I could see it being the case that the advantage of the attacker (being able to advance unseen, being able to advance without being caught in fields of fire by machineguns and artillery, until they are literally in the same hallway as the enemy) outweigh the advantage of the defender (good cover everywhere, defensive positions can’t be spotted in advance and pounded by artillery and bombs) Whereas in a light tunnel network, there are only a few ways to approach so the defender just has to fortify them and wait.
I think a crucial factor that is missing from your analysis is the difficulties for the attacker wanting to maneuver within the tunnel system.
In the Vietnam war and the ongoing Israel-Hamas war, the attacking forces appear to favor destroying the tunnels rather than exploiting them to maneuver. [1]
1. The layout of the tunnels is at least partially unknown to the attackers, which mitigates their ability to outflank the defenders. Yes, there may be paths that will allow the attacker to advance safely, but it may be difficult or impossible to reliably distinguish what this route is.
2. While maps of the tunnels could be produced through modern subsurface mapping, the defenders still must content with area denial devices (e.g. land mines, IEDs or booby traps). The confined nature of the tunnel system forces makes traps substantially more efficient.
3. The previous two considerations impose a substantial psychological burden on attacking advancing through the tunnels, even if they don’t encounter any resistance.
4. (Speculative)
The density and layout of the tunnels does not need to be constant throughout the network. The system of tunnels in regions the defender doesn’t expect to hold may have hundreds of entrances and intersections, being impossible for either side to defend effectively. But travel deeper into the defenders territory requires passing through only a limited number of well defended passageways. This favors the defenders using the peripheral, dense section of tunnel to employ hit-and-run tactics, rather than attempting to defend every passageways.
(My knowledge of subterranean warfare is based entirely on recreational reading.)
As a counterargument, the destruction of tunnels may be primarily due to the attacking force not intending on holding the territory permanently, and so there is little reason to preserve defensive structures.
Thanks! I don’t think the arguments you make undermine my core points. Point by point reply:
--Vietnam, Hamas, etc. have dense tunnel networks but not anywhere near dense enough. My theory predicts that there will be a phase shift at some point where it is easier to attack underground than aboveground. Clearly, it is not easier for Israel or the USA to attack underground than aboveground! And this is for several reasons, but one of them is that the networks aren’t dense enough—Hamas has many tunnels but there is still more attack surface on land than underground.
--Yes, layout of tunnels is unknown to attackers. This is the thing I was referencing when I said you can’t scout from the air.
--Again, with land mines and other such traps, as tunnel density increases eventually you will need more mines to defend underground than you would need to defend aboveground!!! At this point the phase shift occurs and attackers will prefer to attack underground, mines be damned—because the mines will actually be sparser / rarer underground!
--Psychological burden is downstream of the already-discussed factors so if the above factors favor attacking underground, so will the psychological factors.
--Yes, if the density of the network is not approximately constant, such that e.g. there is a ‘belt of low density’ around the city, then obviously that belt is a good place to set up defenses. This is fighting my hypothetical rather than disagreeing with it though; you are saying basically ‘yeah but what if it’s not dense in some places, then those places would be hard to attack.’ Yes. My point simply was that in place with sufficiently dense tunnel networks, underground attacks would be easier than overground attacks.
With modern mobility (air, drones, fast armor, etc.), it’s not clear that “your line having an exploitable hole” is preventable in cities and built-up areas, even without significant tunneling. For “normal” tunnels (such that there could be one every 10 meters in a kilometer as your example), as distinct from “big” tunnels like multilane highways and underground plazas, it doesn’t take much to fortify or seal off an exit, once known, and while it’s not clear which side will decide it’s more danger than help, one of them will seal it off (or collapse it or whatever). Surprise ad-hoc tunnels are problematic, but technology is still pretty limited in making and disguising them.
Note that really effective small, unmechanized and unsupported surprise attacks are only really effective with a supply of suicide-ready expert soldiers. This is, logically, a rare combination.
That said, I don’t much have a handle on modern state-level or guerilla warfare doctrines. So I’d be happy to learn that there are reasons this is more important than it feels to me.
Edit to add: I get the sense that MUCH of modern warfare planning is about cost/benefit. When is it cheaper to just fill the tunnels (or just some of them—once it’s known that it’s common, there won’t be many volunteers to attack that way) with nerve gas or just blow them up, than to defend against them or use them yourselves?
Why would the defenders allow the tunnels to exist? Demolishing tunnels isnt expensive, if attackers prefer to attack through tunnels there likely isn’t enough incentive for defenders to not demolish tunnels
The expensiveness of demolishing tunnels scales with the density of the tunnel network. (Unless the blast effects of underground explosives are generally stronger than I expect; I haven’t done calculations). For sufficiently dense tunnel networks, demolishing enough of them would actually be quite expensive. E.g. if there are 1000 tunnels that you need to demolish per 1km of frontline, the quantity of explosive needed to do that would probably be greater than the quantity you’d need to make a gigantic minefield on the surface. (Minefields can be penetrated… but also, demolished tunnels can be re-dug.)
I think the thing that you’re not considering is that when tunnels are more prevalent and more densely packed, the incentives to use the defensive strategy of “dig a tunnel, then set off a very big bomb in it that collapses many tunnels” gets far higher. It wouldn’t always be infantry combat, it would often be a subterranean equivalent of indirect fires.
Thanks, I hadn’t considered that. So as per my argument, there’s some threshold of density above which it’s easier to attack underground; as per your argument, there’s some threshold of density where ‘indirect fires’ of large tunnel-destroying bombs become practical. Unclear which threshold comes first, but I’d guess it’s the first.
This is not very practically useful to me but dayumn it is cool