It's hard for me to talk about nano-tech, as I worked next door to a nanotech research group for 5 years, and I know where the technology is at the moment.
Robots? Nope. A lever. A pair of crude gears, if you're lucky.
In any event, lets put the goggles of suspension of disbelief and talk about NanoTech.
Nanotechnology was used in the War, as a weapon and as a tool and is now very well established. New materials are built from the bottom-up from elemental building blocks by machines little bigger than cells. And these machines can also be programmed for destruction, instead of creation (both as weapons, destroying organisms from the inside-out, but also as a major player in the breaking down, recycling and re-using of materials). They can also be used to heal, repairing cells individually.