Mike, I’m going to quote at you one of the most original books of military theory I’ve ever read, Lt. Col. Robert Leonhard’s (US Army ret.) Fighting by Minutes: Time and the Art of War (p. 135-136):
It is important to understand the most pervasive and yet frequently ignored axiom of warfare: military forces are perpetually unready for combat. That is, the natural state for a military unit – from an infantry squad to a contingency corps – is unpreparedness.
Most of the time, a soldier is not at maximum alert, weapons ready, focusing his attention in the exact right direction to see the enemy coming: He’s trying to get some sleep on the cold, hard ground, or trying to warm up his unpalatable rations, or taking a leak, or daydreaming about how nice it’ll be to go home, or cleaning his weapon, or digging his foxhole – or even weapon at the ready, eyes wide open, raring to go, and looking in the wrong direction. If it weren’t so, there would be no need to post sentries and change them regularly, since the whole point of a sentry is to have a tiny fraction of your force trying to be alert to danger so it can warn the rest of you to get ready.
What’s more, the higher the level of technology, the greater the degree of unreadiness. A Stone Age warrior just has to wake up, grab his pointy stick, and go. A modern combatant has to keep his vehicle, his sensors, and a whole host of complex weapons maintained, fueled/loaded, and ready, and to stay in communications (through more complex technology that has to be maintained) with other units.
Why is this relevant? Because, Mike, when you picture the deadliness of Iron Empires defenses, and the degree of caution required in movement to avoid being wiped out, you’re implicitly assuming that everyone is perpetually ready, with their fingers on the triggers of those terrifying high-tech weapons. But most of the time, given the fog of war, the maintenance needs of technology, and the limits of human stamina, mental and physical alike, most of the combatants aren’t going to be ready, as Leonhard says.
Simplistic example: A platoon of grav armor blows past at treetop height, about 10 meters, at 480 kilometers an hour. They’re going to traverse from one horizon to another – about 12 kilometers – in NINETY SECONDS. That’s the maximum amount of time you’ll have them in line of sight, assuming completely flat terrain with no obstructions for them to take advance of. There’s no way anyone on the ground is going to engage them successfully without advance warning before they come over the horizon, which means the battle becomes one of reconaissance, counter-reconaissance, signals, sensors, and countermeasures. That’s a battle the maneuvering force has a real chance to win, which means high-speed maneuver is military possible.
Now, Hammer gunships in orbit can cover much wider areas than one guy with an MPIML, and they presumably have a lot more automation to keep them ready a higher percentage of the time. But they’re trying to cover all this ground from hundreds of kilometers up (low Earth orbit being at least 200 km). What’s worse, they’re looking down, not sideways, which makes it much harder to detect an object moving over the surface. So in practice their windows of visibility, in which they have eyes-on a definitive target worth shooting at (and expending energy/ammunition, and making your own position more obvious to anyone watching you) may not be much longer than the 90 second window for that anti-tank rocket specialist in his hole, who is with ten km of his target and probably looking up at it silhouetted against the sky.
So “their only defense is not being seen” for long enough for the enemy to ready, aim, and fire. And that qualifier makes the job of avoiding destruction dramatically easier. You don’t have to make yourself impossible to find (and you can’t); you just have to make the enemy’s job of finding you a bit harder, because it’s already hard enough.
To quote Carl von Clausewitz, “everything is very simple in war, but the simplest thing is difficult.” The fog and friction of war are the reason that armies throughout history have consistently failed to annihilate each other, even in such extremely constrained circumstances as the trench warfare of the Western Front. There’s no reason it’ll be any different in the Iron Empires.