While that’s pretty dystopian, if both borders get that fortified (with miles of no man’s land in between, one assumes) would anyone ever attack? If there’s a Maginot Line facing a... Groener(?) Line, along the length of the border, would there be any warfare?
It would seem so, but not in the way that you'd expect. It'd be infiltration tactics on steroids - small groups trying to get through to the defenses on the over side under the cover of night, or tunneling teams underground trying to sap their way into the enemy tunnel or undermine it to try and cause the bunkers above to collapse, that kind of thing. It'd still be warfare, but it'd be focused entirely on trying to identify weak points in the defensive line, then breach it with a heavy attack. The defender could counter by moving reserves and mobile forces to that area, and so you'd get heavy fighting till either the line is broken through or until the attacker decides to stop and try and find a vulnerability somewhere else.
Rinse and repeat for both sides. The good news is that factories can actually produce bunkers and deliver them to the front line as entirely prefabricated pieces, so you can make large shipments of bunkers by train, then place them where needed behind the main defense line as a secondary, sub wall to delay an attacker until reinforcement can arrive.
They can simply proceed to concentrate this at specific points of the front, wipe out enemy fortification sections one by one at specific parts, then pour through the gap and exploit the vulnerable rear areas.
Isn't the problem there, though, that it is a lot easier to make a huge cannon that stays in one place and keep that well protected than it is to make a huge cannon that can move? After all, whilst the Germans in this scenario could build a railway gun or something of the sort, there's no reason why the French with this super-Maginot couldn't build an equivalent - or even larger - cannon and keep it in a heavily fortified complex, benefiting from the additional space which could be used for better ammunition stores, fire control and artillery plotting rooms, more capable reload mechanisms and other things that wouldn't be practical to put on an already heavy chassis. The Schwerer Gustav, for example, weighed 1350 tons, but that's counting the weight of its entire assembly including the track base, and had a very complicated reload procedure due to the sheer size of the weapon. A fixed version of that gun would weigh substantially less and wouldn't need to be confined to tracks, so it could have a much larger reload mechanism, meaning that it should be able to have a much higher rate of fire.
Fixed weapon positions are basically always more capable than mobile positions, because they've got the benefit of how they don't need to move, which frees up a lot of what would otherwise be serious engineering constraints. You don't need to worry about a bunker causing a bridge to collapse when it tries to cross over it, for example, or about it being too wide to fit on a railcar or go through a tunnel or even just sinking into the earth. A concrete bunker can be fixed into the ground with steel piles to keep it from moving and to absorb recoil, it can be expanded easily if there is the need and, perhaps best of all, they're pretty cheap to maintain after they've been built. The problem is building them in the first place and getting the political will to get them built and keeping it around long enough for the complex to be finished
Reinforced concrete is fundamentally to my knowledge about as strong as it has always been,
Nah, concrete has gotten a lot stronger since the past. Concrete from the 70s crumbles at 3500 psi, modern concrete can go much further than that, especially if it is the prestressed kind. That stuff can go to eight thousand psi, absolutely dwarfing anything you'd find in WW2. Concrete from the 20s was even weaker on average, between 500psi to 1000psi from what I know, but that might just be the samples that are like that.
And nukes of course, are an atomic problem solver that there is no real good response to.
To be fair, that solves pretty much all problems, though
Here's the way I could foresee such a line being structured: a heavy, Maginot-esque superfortification on the national border, backed up by number of fast response mobile units able to rush towards any breach in the defense wall and close it before the attacker can properly exploit it, as well as to relieve any sections of the wall that are under serious attack. Concealed, ground level AA posts are spread across the lines, equipped with quick firing autocannons (37 to 40mm) to protect against low flying bombers that would have the accuracy to hit turret and bunker roofs, which themselves would either be reinforced to better resist direct attack or concealed with a sheet of grassed earth over the top to make them nigh impossible to spot from the air. As technology progresses, this line would first be upgraded with radar posts to provide early warning of enemy air attacks, then SAM sites to replace ballistic AA, then culminating with something akin to the Israeli Iron Dome for shooting down missile strikes and protecting any exposed vulnerabilities in the line itself.
That creates a line that modern technology is more or less incapable of breaching without escalating to nuclear weapons as you're facing one of the following situations:
a) You can't actually find the target because it is concealed, so you can't attack it in the first place.
b) You can't use artillery to snipe the defense posts, because the defender has similar guns and has the benefit of being able to encase them in fixed positions, whereas the attacker has to bring them up...with all the engineering problems that entails.
c) You can't use aircraft to bomb out the defense positions because of a combination of point A and the amount of ground fire that they can receive...and even then, airstrikes are going to be of limited effectiveness as there is a serious limit on the amount of explosives a plane can carry before you need to spend months designing an entirely new airplane and retooling the production line to build it in numbers, whereas it is much, much easier to just reinforce the bunker wall itself...and where said modifications to make the plane better suited for taking out said positions in the first place is going to render them even more vulnerable to interception.
d) You can't use missiles to bomb out the defense positions either, because the ground base on the defender's side would be able to shoot down the cruise missile before it reached its destination, and being themselves in a fixed position, would be able to have enough ammunition on hand to intercept an entire barrage of fire without too much difficulty, protecting the static positions from easy destruction.
e) A nuclear attack on the line would likely be met with one in kind upon the attacker, meaning...well, bad things all around. At least they'll have bunkers to keep the fallout away, though.
You'd need a
radically different political climate to get this stuff built, though, and quite a bit of cash to build the wall up in the first place. Armies have the benefit of being able to, y'know, go on the attack. Concrete bunkers aren't nearly that good at attacking as they are at defending
Still a fun thought experiment to be sure!
