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?
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?