The line is blurry now...but Marines were originally snipers in the rigging of the tall ships. Fun fact that is why Marine officers have braiding on the top of their covers. The more decoration the higher the rank.
All of y'all may be right OK I just prefer to call any military personnel soldiers out of respect for their service, sue me I am ready for some football done with this discussion soldiers.
Space Force - i propose "Astros" for the regular enlisted. Maybe the elite fighting division that makes first entry to a planet or moon, etc....could be called the "AtmosGuard"? Not sure.
This is quickly turning in to beating a dead horse....the only comment left for me is to say that it's not about what YOU prefer. It's about understanding and respecting THEM. You'll just have to trust me on this, but NO Marine will feel like you have respected them or their service if you call them a soldier. Likewise, a Navy pilot/USNA graduate will not feel respected if you call them a soldier. They each have pride in what they do and if you lump them all together, that isn't showing respect. Good fact, thank you. I still don't see a blurry line. On the base, it is very clear to me that Marines are different and they believe it.
The description of what Marines do and what the Army does have been blurry since Vietnam. Marines are supposed to hit hard punch a hole and let larger better equipped forces continue the fight. The Marines don't have or at least didn't have when I was in a logistics base large enough for sustained combat. What you saw Marines do in the first gulf war was mostly traditional. They got there in large numbers quickly, punched a hole in the itaqi lines took Kuwait City but didn't travel far in land. Second war they traveled across land all the way to Bagdad and then stayed there as a police force. That's is a traditional Army task. That is what I mean by getting blurred.