Miniature wargaming in the Solar System.
The background is that of the TV series "The Expanse", but I have branched off with a totally alternative ending to the first half of Season 3 !
This is hard and dirty Sci Fi : the fundamental laws of physics hold (no travel faster than light, no anti-gravity, and so forth), and technological progress has made no fundamental changes to human society : poverty and suffering are rife and war is always just around the corner.