So the Rage war trilogy takes place in one of the furthest flung time periods of any of the stories in the shared franchises. It's a time in human history that's been beset by several species wide cataclysms in the half dozen centuries since humanity became spaceborne. By this point we have tiny pockets of human presence spread over a vast area of mostly nothingness in a sphere with a roughly 500ly radius surrounding the Sol system, there's a few big worlds out there but mostly it's these piddly "truckstop" stations and outposts along the vast empty desert of night. The picture Lebbon paints of humanity at this point is mostly unified and perhaps split more along corporate afflictions than any kind of national affiliations so a lot of the grunt work of previous state run tax funded military is handled by corporate security and other free contractors.
Despite the physical sprawl of our reach our populations are thin and even moreso are the military forces needed, and even available to service that sprawl. And like the previous posters said, there's a lot of technology picking up whatever slack there might be. There's little to no mention of large scale interhumanity conflict, there doesn't seem to be much individual scales large enough to allow it, and naively or not, there's very little contact and thus requirement for defense against non-human cultures in the space we have reached out across.
