They are under different regulatory bodies:
They are indeed, but that’s not the reason why surgeons aren’t called Dr. The Royal College of Physicians also only represents hospital drs, not GPs.
Which is further supported by the fact that there is no Marine Corps service academy. Graduates of the Navel Academy have the choice of becoming commissioned officers in the Navy or the Marine Corps. In fact about 1/3 annually choose to become Marine Corps officers.
Because of convention, although it’s not absolutely followed by everyone.
As an example, the National WWII Museum seems to follow the convention capitalizing “Marine” and not “Soldier.”
The museum is famous among WWII buffs, holding conferences, publishing articles
and hosting guest speakers whose lectures are posted on YouTube.
While they are not the final arbiters of style, their style reflects the usage by historians when discussing WWII.
Note the capitalization of “Marines” and none for “troops” despite the fact that the troops were US forces.
Note that both “soldiers” and "troops are lowercase, again despite the fact that the troops were US forces. Also note that “forces” are is also not capitalized.
If this were a discussion among linguists, I’m in the descriptivist camp. Prescriptivists can argue whatever they would like, but this is not what is actually used.
Also note that the convention of chapitalizing “Marines” in not universally adhered to, but I do not recall “soldiers” being capitalized.
My WAG is that “Marine” and “Marines” when used to describe the troops are usually capitalized because they are closely related to “the Marines”, the institution, where “soldier” and “soldiers” is a completely different word than “the Army”, meaning the US Army.
And that marks the end of my participation on this side discussion.
As was noted upthread, the New York Times style guide and the Associated Press style guide both dictate the capitalization of “Marine” but not “soldier.” This is likely why historians typically follow the same pattern, as you point out.
However, the various branches of the military are the ones dictating that “Soldier,” “Sailor,” “Airman,” etc. be capitalized in their official communications. I was simply pointing out that if you are going to capitalize “Marine,” why not capitalize all service members for consistency and like their service branches apparently prefer? It seems a little odd that only “Marine” would be singled out for capitalization by convention.
Finally, I’m not aware of anyone advocating for the capitalization of “troops” or “forces”.
Do you believe that the style requests from all organizations be followed, or just ones from the US military?
As said earlier, I’m following the same style as history books, as I’m used to it.
Early 1990s, I’m at an indoor soccer match to watch my cousin, a new Marine, play. Just a few minutes into the game he breaks his nose and we drive to some military medical facility for him to get treated. Naturally, we are having to wait.
I, about 12 years of age, had heard my cousin disparagingly refer to the Navy as “squids” in the past, so I made some comment about how we were having to wait because of “all these squids”.
I guess that’s a real insult, because I remember him telling my to be quiet, considering that Navy doctors were the ones treating him. He seemed pretty serious.
It isn’t a real insult, no more than jarhead. They’re ribbing terms and that is all.
Now “Self-propelled sandbag” might not go over as well with some Marines. OK to use it among themselves, but us Squids and Snipes need to be more careful about using it, never mind Army or civvy.
Yeah, it’s just some mild ribbing and all in good fun.
That being said, Moriarty does explain that his cousin was a new Marine and you really shouldn’t be using them if you’re some fresh-out-of-boot nub.
IME, squid is a bit derogatory while jarhead not as much. People have tried using jarhead in a derogatory manner but it isn’t so bad. Now if one said dumb jarhead our stupid jarhead, obviously that’s different.
In the broken nose story relayed by @Moriarty , the guy was there for some care and medical treatment so you want to avoid using squid in that scenario. You’re at their mercy, and how you’re treated can be more painful if they’re not careful. Yeah, don’t piss them off. Basic common sense.
Yeah, I learned early in my navy career that if there are Marines around you don’t mess with the cooks who feed 'em, the disbursing clerks who pay 'em, and the corpsmen who patch 'em up.
This is certainly not specific to the U.S. military. I’ve mentioned upthread that I have followed similar style requests from Scouting America (formerly Boy Scouts of America). But I’m a U.S. military veteran and a current Scout leader, so that perhaps makes me more inclined to follow the style requests of these particular organizations. (Although speaking as a veteran, it does also smack somewhat of pandering.)
And while “all organizations” is a pretty broad characterization, I think I would have no problem in general following various organizations’ style requests if I am made aware of them.
Fair enough, that is certainly your prerogative. And while that is indeed the style of various news organizations (and “history books” according to you, although I’m not aware of any particular “history book style guide”), it is inconsistent if you think about it. Why is it that only “Marine” gets capitalized?
For what it’s worth, according to this blog post from the New York Times, that news organization only started capitalizing “Marine” in 2009!
A good question. I capitalize it but don’t capitalize soldier or sailor or airman. As to why that is, I have NFC, not the foggiest clue. 
It depends how difficult their requests are to follow. When the Mormons said they wanted to be called the Church Of Latter Day Saints, i said, “okay”, and referred to the organization as the Church Of Latter-Day Saints, or more often, the LDS Church. But when they said, “no, that’s not good enough, we are the Church of Jesus Christ of Latter-day Saints” , i said, “screw it, I’ll just call them Mormons.”
Oh! You missed the most important part!
The name is the Church of Jesus Christ of Latter-day Saints.
They ask:
They initially asked not be be called Mormons some time back in the 00s. Then suddenly they spent millions of dollars promoting that name in a TV campaign from 2010 to 2018.
I grew up Mormon. I see no reason to follow that request.
(Also note that there is a difference between:
The Church of Jesus Christ of Latter-day Saints
The Church of Jesus Christ of Latter-Day Saints (can you spot the difference?)
The former are the ones in Salt Lake City and the latter was a branch of Mormonism which died out.)
I think i said that
There was a period when Mormons i interacted with preferred the Church to be called the LDS Church, rather than the Mormon Church, and i tried to do that. It’s when they added “Jesus Christ”, and made the name really long and impossible to abbreviate in an easy way.
I mean, the full name always included “Jesus Christ”, but the leadership made a fuss about how that was important to include whenever talking about their group. That’s when i gave up.
I was already out when the initial request came to stop using “Mormon” but I believe they preferred to be called by their full name, including Jesus Christ, but allowed the abbreviated LDS Church.
Individuals may have been different but the church itself would never have allowed The Church of Latter-day Saints.
Of course, I was already out by then. The only Mormons I talked to were my mother and other relatives, and a lifetime friend. For all of these, I just said “the Church”, because that was what Mormons use when talking to each other.
Shhhhh. Or else you’ll get Hegseth and the anti-PC crowd all down on them for “identifying” as capital-S Soldiers and capital-M Marines.
Seriously though; that might be a fantastic argument or analogy for how people identify and why it’s not politically correct BS.
Good point. Within the military, there’s nobody more insistent on their identity than a U.S. Marine. They even get annoyed if they are referred to as a “former” Marine—even if they just did a four-year active-duty stint decades ago.
I made that comparison above: