Among science fiction stories with space flight, the overwhelming majority are about combat, both between spacecraft and between futuristic ground troops. Not to mention the occasional starship marine assault trying to board a hostile ship while in flight. Yes, there are a few non-combat stories, mostly about exploration, but space combat is here to stay.

This is just the natural continuation of the process of militarisation of space

Which naturally leads to questions about the space branch of the military of various nations. The “astro-military” in other words. Some may start out as a subdivision of an existing branch and eventually grow large enough to split off (such as how the US Army Air Corps spit off to become the US Air Force in 1947). Some may grow large enough to absorb other branches of the military, others may be reabsorbed into other branches. In William Keith’s Galactic Marines series one of the themes of the early novels is how the US Marines fight being absorbed or eliminated. Their solution is diversifying their mission to include performing assaults on Luna and Mars.