So, my world is a sphere (roughly) within hyperbolic space. Since the surface areas of spheres in hyperbolic space grows exponentially with radius, it has the same surface area of the Earth (indeed, we can assume that the surface is the exact same as the Earth (continents, countries, and all) for the purposes of this question), but only has a radius of 4 km (2.5 mi).
This means that you could, for example, start in the US, dig a 8 km tunnel, and end up in China. Indeed, you can reach any point from any point with a 8 km tunnel (ignoring the heights of mountains and stuff).
In the past, the hyperbolic nature of the universe has not affected the world, since humanity was stuck to the surface of the sphere (spheres in euclidean and hyperbolic geometry are the same), but towards the beginning of the twenty first century, the inhabitants started to be able to build tunnels through the world.
This of course has awesome economic potential, but many militaries are worried. How do you stop enemies from digging under you country? How do you stop enemies from digging into your base, or into your country, to either deploy troops, plant bombs, or even just dig a giant pit under you, your buildings, and your land?
Remember, they don't need to be next to you. Enemies anywhere in the world with the digging capability can easily get to you from below. Indeed, even if they are next to you, going through the center of the world is probably quicker. Not only that, but by going below, they can hit you anywhere, not just the border.
It may be hard to visualize how a sphere with the surface area of earth but only a 4 km radius would even be possible. To do this, it might help to visualize it within the Poincaré ball model, centered on the world. Note how circles grow exponentially with respect to radius in this model, instead of quadratically. For example, look at the heptagon tiling. A large circle of heptagons has a much smaller diameter than you would expect (note that all the heptagons are the same size).
Once countries realized the economic, and more importantly military implications of the situation, they invested very heavily in digging and tunneling technology (similar to how on Earth, we invested heavily in nuclear technology). As such, tunneling technology is particular will be much more advanced than in our world. In particular, I imagine they might have developed "worm tanks", i.e. vehicles/drones that move through the ground without needing a tunnel.