 They first appeared in the short story "Combat Unit" (1960), and have since been featured in science fiction novels and short story anthologies by him and others.The story of the books takes place in various times from the near-future (2018, 2068), the mid-range future (27th up to 37th century) and even farther in one case (118th century).The overall plot features mostly military themes and includes space exploration, alien races and some advances in human society.