In my setting the world is practically covered in dense forests, mountains or mountainous forests. As to they whys, for now its easier to just say that's the way it is. Technology in this world is near to farther future if that makes sense. So things like ETC, CLG, and railguns (can only scale down to what is basically a trailer truck with a cannon mounted onto it, limits are at energy storage and generation) exist and are in use. The weapons are more than capable of hurting our modern day MBTs due to the significant higher muzzle velocities, of course traditional weapons with far lower velocities exist. Active point defense systems are mostly reliable and in use. Experimental armor types such as electrical armor and a very limited and early development field effect shield system exist. That for all intents and purposes can re direct incoming munitions at a slight deflection angle to try to force a miss if lucky. Drones/ parasite machinery is not necessarily a concept anymore, rather using drones for information gathering or use as defensive bait are in common use. Basically technology is advancing at a fairly reasonable pace
The forest and landscape however is limiting. The dense forest has a wide variety of fauna, some trees prove to be incredibly tough to break down leading to a situation where there is sustained forestry during the fighting (simply burning down a forest completely isn't a solution, though weaker trees and fauna can be chopped down). Forest canopy is at a high elevation, but for all intents and purposes sending munitions through the top of the forest isn't possible; so arcing artillery platforms are challenging and is a central point (basically fighting is confined to the forest and aerial/long range artillery support isn't really possible). Mountainous regions are also open or covered in thick forestry as well. Parts of forests can be toxic or caustic to humans without adequate nuclear, biological or chemical systems (NBC). Others can interfere with radio signals or interfere with sensors. As for if there's maneuver warfare or dug in, I have not entirely decided that yet.
Suppose two peer forces are stuck fighting in what is essentially the mountains or plains of Afghanistan, and the forests of Vietnam combined. What kind of combat vehicles would arise in a conflict where both sides were forced to fight in such an environment (leaving or ignoring the fight is just not an option). And for plot reasons the entire machine cannot be remote controlled or AI based (forest interference, enemy ecm, the thick forest itself etc.) Humans should ideally be in control of it, how much though doesn’t really matter and the help of advanced systems to automate processes are more than fine.