[ericthered]

Originally Posted by Apollonian

One immediate thought is a dungeon world - it's quite hard to push a tank through a 10'-wide stone hallway. It's the kind of thing that could work if the only portals are accessible through dungeons/caverns/otherwise difficult spaces.
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I would probably approach the bullet problem from a couple angles. Spell effects, as above; monsters that are resistant to piercing damage, like undead and blobs; and terrain that usually has short sightlines, so that it's possible to close to melee range. Importantly, none of these are insurmountable problems: spells can be counterspelled, resistances can be overcome with volume of fire, and firearms are still handy in close quarters.
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Mind control effects are going to be a big whammy, but it's boring if they're a never-fail win condition. I'd definitely include them (if for no other reason than I think mind flayers make great bad guys), but there would need to be workarounds. Experimental psychotronic stuff that doesn't always work, for example...

A classic D&D underdark setting might be a good baseline: extremely rough terrain, no sky to fly in, mystically powerful inhabitants and deadly freakish wildlife.

I'd personally include a few insubstantial foes that can strike with impunity, at least until some countermeasures are obtained from locals one way or another. Another answer to bullets might be foes that come back from the dead. This includes cursed locales, but it also includes the locals casting resurrection spells, and some monsters with fun regenerative properties.

The very easiest GURPS answer to bullets is an abundance of Missile Shield and Reverse Missile effects, but that also feels a bit like cheating. It's the kind of thing where I'd put in a countermeasure that's possible, but costly - individually enchanted ammunition, where putting it on a dozen arrows isn't too bad, but several magazines of 5.56 NATO get prohibitively expensive.

I'm curious just how effectively opponents would be able to spam those spells. The magic isn't nearly as powerful in its original context, it costs a lot to cast, and you need to know that its the spell you want. If it can only be kept up for a few minutes at a time, it adds a strategic element to battles.

Another challenge is logistics: a Modern Army depends on a good and reliable supply chain for ammo, gas, and food.

Originally Posted by SilvercatMoonpaw

A world of islands floating in air might also give modern militaries pause (I think). Not as much as a dungeon-world, but I can imagine having to use mostly planes and maybe blimps would cut into the usual logistics.

Originally Posted by Agemegos

That suggests that a Narnia-type difference in the time rate might do it. If a royal career of several decades in the portal realm passes unnoticed in a mundane afternoon, then keeping a battalion in supply through the portal is going to be an economic challenge. It will consume more supply than the entire US armed forces, and delivering so much everything to Professor Diggory’s spare bedroom would be challenging. Whereas any troops that fall back to the mundane world to resupply will return to discover that not only are the battle and war lost, but that they are forgotten in the deeps of history.

Logistics is an interesting question. On the one hand, modern militaries rely heavily on it. On the other, they've put a LOT of effort into being good at it.

I suspect dungeon world gets roads blasted into it pretty quickly, and those will be useful... you just have to build a road to get your cool stuff anywhere.

The air world would get slowed down by needing to fly everything via helicopter... but ultimately airlift is something the military practices and is fairly good at. They've even worked out how you adjust equipment when you have to airdrop everything.

The narnia time difference problem is weird, and a lot depends on the relative time rate. If its something sanely managable, like 20:1, you just have to supply the folks inside 20 times more. Its annoying, but really just a problem of big numbers. If the number is something like 1000:1 or worse, you probably need to throw an expedition at it you know won't fail and has everything it needs for the campaign. And you might want to send a chunk of manufacturing.

Originally Posted by Inky

Another possibility is to just make it a not very powerful effect that can be resisted by a Will roll, IQ roll, or something like that. Less like Professor X, more like the fairy music that lures people to their doom. Then, since high-tech equipment doesn't make any difference to that (unless you introduce some made-up psychotronic equipment that does), your commandos are faced with exactly as much of a challenge as King Arthur's knights would have been. Not being King Arthur's knights, though, they may have different ideas about how to get past it.

Hmm, if they tried to overwhelm that with numbers in the hope that some might get through by luck, it might be an interesting thing to have some kind of magic which has more effect the more people are present - a fear spell where everybody amplifies each other's panic, for instance. Then you can't send in your army and the PCs have to try and force their way through alone.

Magic that forces elite forces is a really good thing to encourage. Foes that transform their victims into more of themselves are good for that. Like a necromancer who only needs more bodies to make more undead, or some sort of aberation that seeds its victims with eggs. Mass anti-army fear spells could work too.


One thought is having a local "I win" button that has a local countermeasure that's in limited supply, like magical amulets or swords that can hit ghosts.