The world
Only the product glows
Veyholt Materials built a swarm of self-replicating disassemblers to pull every useful element out of scrap and rock, and condense it into crystal. One harvester load of condensite replaces a mine, a smelter and a supply chain.
Then the swarm got out, and found that living matter is the easiest matter in the world to take apart. Forests went first, then livestock, then towns. Where it passes it leaves a bloom: a field of glowing blue spires where something used to grow.
The war is fought on the Verge, the moving edge where the Bloom meets living land. Fresh crystal is the richest there is, which means it is richest exactly where people still live.