I'd like to ask for some help from either ex-military or modern military buffs to help me nail down some descriptions of the types of mission forces assigned in standard, modern military.  Here's what I have for the big three thus far:
   Army – Armed forces consisting of infantry, artillery, armored cavalry, and airborne cavalry units.
Marines – Amphibious armed forces organized for rapid deployment, consisting of infantry, artillery, armored cavalry, airborne cavalry, special forces (“commando”), and air support and strike units.  Commonly noted for their elite training and esprit de corps.
Navy – Armed forces consisting of naval support, naval artillery, amphibious special forces, intelligence, and air support and strike units.Please, critique content, wording, nomenclature, whatever.  I'd like to get this right.  Succinct, but right.

!i!

So...did I nail it right on the first try? Or is this just not the right place and time for this query?

!i!

Not an expert, but the Army too has special forces.

-clash

Quote from: Ian AbsentiaArmy – Armed forces consisting of infantry, artillery, armored cavalry, and airborne cavalry units.
Marines – Amphibious armed forces organized for rapid deployment, consisting of infantry, artillery, armored cavalry, airborne cavalry, special forces ("commando"), and air support and strike units. Commonly noted for their elite training and esprit de corps.
Navy – Armed forces consisting of naval support, naval artillery, amphibious special forces, intelligence, and air support and strike units.

I've never been in but I've worked for the Army, Navy, Marines, and Coast Guard (sorry never Air Force) for 9 years.  I'm assuming you're only interested in what DOD refers to as the "warfighters" and not the support roles.  You'll find that the services have a variety of roles they fill that often overlap each other.  It's hard to generalize but you have the basic structure down.  If you have specific questions I'll try to answer if someone doesn't.

Army - Generally considering an occupying force.  They do indeed have Special Forces (Green Beret), Rangers, and Delta Force.  Special Forces are also rapidly deployed.  
Navy - Probably the quickest asset to provide presence in theater.  A carrier group alone is huge.  Also, submarines, the boomers, have a large arsenal of tactical nuclear warheads.  You'll usually hear the division within the Navy referred to as surface, aviation, and sub-surface Navy.  They also have special forces (SEALs).
Marines - Actually fall under the Navy, just don't tell them that.  They don't have special forces but do have specialized training schools like Recon and Sniper.  The reason you hear they're the first in is because the Commander in Chief does not have to get congressional permission to keep them there for over 30 days.  The Army does other than their Special Forces.  Very small, very efficient and usually deploy with the Navy presence.
Air Force - Air Superiority.  Except for their long range bombers, they usually will end up setting up an airfield in theater to assist the Navy's efforts.

Good, good.  Thank you for the reply.  What I'm trying to do from the perspective of Traveller is to provide a one or two sentence description of the roles that one might expect to serve upon enlistment in any of the mentioned careers, not necessarily the over-arching mission statement of the branch itself.

A couple of notes worth mentioning:

I didn't mean to slight the Air Force by not mentioning them.  Well, I purposefully didn't include them, but it's because they don't figure directly into the purpose that I currently have in mind.

Regarding special forces, this is the reason you never rely solely on Wikipedia as a research source.  I referred to Marine special forces (lower case), intending their Recon and Sniper schools, and not Special Forces (upper case) as in a unit designation.  So they aren't organised into discrete units?

Also, I was aware that the US Army has Special Forces units, but I didn't mention them in the context of Traveller because I was looking for a way to distinguish the Army from the Marines in describing their typical role.  However, it's probably not best to create distinctions where none exist when I'm trying to model a real-world military career.

As far as "warfighter" versus support roles go, I probably ought to include support as well.  I was just trying to keep it brief.

And finally, I'm interested in not just the US armed forces, but any modern military organisation.  Like the original Traveller, I'm relying heavily upon the US armed forces as a primary model, but any other nation's military will prove interesting and helpful as well.

Thanks!

!i!

Here's my revised entry, including the civilian careers for context:

Army – Main body armed forces consisting of infantry, artillery, armored cavalry, airborne cavalry, and special forces units.
Marines – Amphibious armed forces organized for rapid deployment, consisting of infantry, artillery, armored cavalry, reconnaissance, and air support and strike units.  Noted for their elite training and esprit de corps.
Navy – Maritime armed forces consisting of surface, subsurface, and aviation divisions.  Also includes amphibious special forces and intelligence units.
Merchants – Civilian maritime organizations involved in the contracted transport, and sometimes purchase and resale, of material goods, and the operation of associated vessels.
Scientists – Civilian organizations involved in academic and commercial research and application of knowledge.
Other – A catch-all category for just about everyone else, often indicating an individual who has held no single, steady career.

Again, comment, content, and critique are welcomed.

!i!

The four core elements the Marine Air-Ground Task Force are :

The command element (CE), a headquarters unit that directs the other elements.

The ground combat element (GCE), usually comprising infantry, supported by armor (tanks), and artillery, but may also include special units such as scouts or Force Reconnaissance, snipers, and forward air controllers.

The aviation combat element (ACE), which contributes the air power
to the MAGTF. The ACE includes all aircraft (both fixed wing and helicopters), their pilots and maintenance personnel, and those units necessary for aviation command and control.

The logistics combat element (LCE), contains all of the support units for the MAGTF: communications, combat engineers, motor transport, medical, supply units, and certain specialized groups such as air delivery and landing support teams.

This is one of those things that gets more complicated the further you look into it. Suffice to say, there is a lot of overlap within all of these groups (and every single group absolutely knows for certain that they are the best at doing it!).

- Q

Ian, are you looking for planetary groups, or stellar/interstellar ones? You seem to be veering towards planetary, but I'm not sure, given the 'Traveller' tag in your topic.

- Q

if you want it in the traveller context, keep it really simple:

army:  ground troops, main fighters, occupiers if need be
navy:  controls the space between planets/stars, helps ground forces via orbital bombardment
marines:  ship-board troops; carried to the theater of ops by the navy for special operations.  also in charge of ship-board defense and boarding parties.

that's about it, really, as far as the imperium goes.  you could keep adding additional functions & missions to complicate matters, but them's the basics.

Quote from: QuireIan, are you looking for planetary groups, or stellar/interstellar ones? You seem to be veering towards planetary, but I'm not sure, given the 'Traveller' tag in your topic.

Planetary, specifically contemporary Earth -- the modern world in which we live.  Thus the "-esque" modifer to the Traveller tag.  Also, you and beeber are quite right -- this is like an onion where the rings seem to get bigger the more layers you pull away, and it's best to keep it dead-simple if you want to preserve that classic Traveller-esque economy of style.  I'm trying to strike that balance between being informative and being brief.

In case anyone is wondering, this is related to my Mariner project.

!i!

Quote from: Ian AbsentiaIn case anyone is wondering, this is related to my Mariner project.

Ah right. Then I think you've pinned it down quite nicely.

- Q

Beware that force is not equal to expertise.

So there might be people in each force that have the exact same skills. Logistics personnel or maintenance people, or clerks.

Quote from: Ian AbsentiaMerchants – Civilian maritime organizations involved in the contracted transport, and sometimes purchase and resale, of material goods, and the operation of associated vessels.
Scientists – Civilian organizations involved in academic and commercial research and application of knowledge.
Other – A catch-all category for just about everyone else, often indicating an individual who has held no single, steady career.

You could sum these professions up with one word...contractors.  

Quote from: GunslingerYou could sum these professions up with one word...contractors.  

Yes, but why would you want to?

-clash