So this question is tough - not just because of the 20-Year Rule, but also because the question surrounding the Marines has existed for quite some time with no easy answer.

I should point out that Marine forces still exist in many major militaries like the UK's Royal Marines and Republic of Korea's Marine Corps (ROKMC).

What is massively disproportionate between the USMC and other Marine forces is size:

• The US Marines have approximately 184,400 active personnel per table A.2, with 38,900 in reserve.
• The Royal Marines, however, number only around 8,000 total.

So what makes the USMC unique compared to other Marine forces? Well first, let's see what their mission statement is:

As America's expeditionary force in readiness since 1775, the Marines are forward deployed to win our nation’s battles swiftly and aggressively in times of crisis. We fight on land, sea and air, as well as provide forces and detachments to naval ships and ground operations.

Marines have a long history of developing expeditionary doctrine and innovations that set the example while leading other countries in multinational military operations. These unique capabilities and leadership qualities make the Marines our nation’s first line of defense.

So OP touched upon this, but the Marines have been around since 1775 as naval infantry and from that the tradition as an expeditionary force in readiness has grown and evolved.

Indeed, while airplanes have made it possible to move people around the world quite fast, ships still maintain a key role in moving heavy equipment around the world - as the Military Sealift Command demonstrated in Desert Storm, moving more than 12 million tons of equipment - and so an expeditionary force in readiness that wants to fight with sufficient supplies AND heavy equipment means being closely married to ships and thus naval forces.

I carefully mentioned those particular characteristics because in warfare, it isn't enough to simply have people available, but you need their equipment and sufficient supplies to conduct any form of operations. As thus, the Marines have built up a model of almost entire self-sufficiency (relying heavily on the Navy for some areas, as part of their unique heritage and relationship together).

At the core of this is the MAGTF - Marine Air-Ground Task Force - because ever since WW2, it's been clear that ground forces without sufficient air support are at a distinct disadvantage. In addition, at Guadalcanal in 1942, when the US Navy was battered and withdrew, Japanese warships were able to bombard Henderson Field and the Marines there on November 13-14. Supplies on the island were also limited and air cover from Navy carriers became limited.

Thus you again see the three core tenets of Marine self-reliance: ground forces, air forces, and supplies to sustain.

Today we have the Marine Expeditionary Unit - the smallest of the MAGTF's - and is comprised of about 2,200 Marines and sailors. Their composition is listed here:

• Command Element - 200 personnel
• Ground Combat Element - 1200 personnel - a standard infantry battalion designated "Battalion Landing Team" when assigned to a MEU and augmented with amphibious assault vehicles, tanks, artillery, combat engineers, etc.
• Air Combat Element - 500 personnel - provides heavy and medium lift, assault support, close air support capabilities through the CH-53E Super Stallion, MV-22B Osprey, AV-8B Harrier, UH-1 Huey/AH-1 Cobra helicopters, and eventually the F-35B Lightning II
• Logistics Combat Element - 300 personnel - allows the MEU to sustain itself in austere environments for 15 days

So again, you see that the MEU is comprised of those 3 key elements: ground forces, air forces, and logistics for self-reliance.

These scale up to Marine Expeditionary Brigades (MEB) and Marine Expeditionary Forces (MEF) which all grow in size (the ground combat component of the MEB, for example, is a regiment) and length of self sustainment (MEB has a 30 day requirement). You can read more about it here in the USMC's Expeditionary Force 21 document.

So in these regards, the USMC is quite unique and different from the Army (which lacks air forces and isn't expeditionary focused), Navy (which lacks ground forces), and Air Force (which lacks ground forces and expeditionary focused).

Don't discount the role tradition plays either in how the US organizes its military forces.

If you go back to the DOD Budget Request for FY2018, in Table A-2 and A-3, you'll see how the US military splits its personnel between Active and Reserve/National Guard Forces:

Branch	Active	Reserve/Natl Guard	Total	% Active
Army	476,000	542,000	1,018,000	47%
Air Force	321,100	174,700	495,800	65%
Navy	326,500	58,200	384,700	85%
Marines	184,400	38,900	223,300	83%

Notice how both the Navy and Marines have a >80% of their total strength coming from active duty personnel. Both are expeditionary focused (after all, US naval warships sail far and wide across the world) and considered rapidly deployable in contrast to the heavily Reserve/National Guard reliant Air Force and Army.

As thus, the Navy and Marines have a reputation of being America's '911 force' due to the lack of need to activate/recall people in the reserves or National Guard to active duty and due to active duty forces being ready/necessitating less turn around time to deploy than say, a National Guard unit that hasn't trained together for some time.

Now why do I say tradition plays a role in all this? Well, historically, the US Army was small and heavily reliant on conscription and state militias in times of war. Peacetime Army forces were small, and as thus, whenever naval forces overseas needed ground forces - which could come at anytime - they relied first on Marines already assigned to said ships, as in the case of the Barbary Wars.

That tradition has evolved with changes in warfare and threats around the world, of course, but even today you can see that there are very different philosophies between the Marines and Navy with the Army and Air Force, for instance.

There are definitely legitimate questions about the Marines though. As I mentioned above, the Marines aren't entirely self sufficient: they still rely on the Navy for the ships that carry them, as well as Navy medical personnel (everything from doctors to corpsmen are Navy personnel) and even chaplains.

So some have pondered certainly whether major changes to the Marines could come. For instance... the Navy's flight school already trains all Marine aviators - so why do the Marines need their own air assets? (As someone once said.. 'why does the Navy's Army needs its own Air Force'?)

Could the Army have units in readiness to act as ground forces on naval warships?

Of course, all these questions rapidly hit the realm of politics, where the popularity of the Marines with the general public as well as very real operational considerations (both the Marines and Navy fall under the Department of the Navy, reducing a layer in the chain of command and have a close history of working together) make drastic changes tough and take this conversation well out of the scope of this sub.

Hi everyone,

This is just to remind all those who'd like to respond of our 20-Year Rule. Please discuss only the history of the USMC up to 1997 - not its current strategic role or deployment.

Thank you!

The Marines have always feared being absorbed on some level by the Army. There were several "crisis" points in which this appeared somewhat likely in the last 100 or so years:

1. Post World War One, while the Marines had performed well, they had also basically functioned under an Army command structure (chiefly as part of the US Army 2nd Infantry Division) which in a time of great downsizing, opened the question to if there was really a need for a parallel ground force separate from the Army.

The answer to this was two fold:

The USMC focused very heavily on building an amphibious doctrine to allow for land force projection over sealanes. It's worth keeping in mind to this point most "amphibious landings" were just that, landings, troops disembarking on a open beach, while the Marines were increasingly interested in what we would view as "amphibious assaults."

Further the Marines actively pursued a role in literally any minor action that rose up in the interwar years. It's why the various "Banana Wars" were largely carried out by USMC forces.

In this the Marines basically secured a "Small wars" proponency the Army did not want, and a sort of relevancy the Army did not have in regards to small force projection in both doctrine and mission, which is likely why we arrived at the start of World War Two with Marines at all.

2. After World War Two, the same discussion was revived, less so because the Marines had not earned their keep in World War Two, but in the nuclear centric post war policy and massive cuts to the ground force component, the role of a independent Marine Corps was questioned (it's worth keeping in mind the Army proved equally adept at the "Marine" amphibious mission during World War Two).

The Marine answer in many ways was to again, build themselves a niche, and stake out dominion in missions the Army was either disinterested in (USMC's "World War Three" focus would have been on the periphery in Norway or potentially CENTCOM AOs vs the "Main event" in Germany/Central Europe) historical strengths (amphibious landings), while maintaining some of the small war proficiency/focus (see the Dominican Republic, Beirut etc).

Then pair this with very aggressive marketing (the Marines opened World War Two with hiring a PR firm). I'm not going to go into it at length, but the USMC is very aggressive in engaging with the US Government and populace, and establishing a public consciousness of the "need" for a USMC, or superiority of USMC assets. In so many words they've wrapped themselves in the US flags to become part of Americana by far more than the other branches of the military have.

By the 1990s though, this smaller, more forward focus resulted in the following sort of distinct capability:

The Marines focused heavily on "light" infantry, forward deployed on a rotational basis aboard ship, that could be augmented as required to meet missions. This is actually quite useful, as the US Army of the Cold War to 1990's era basically came in two flavors:

Light Paratrooper type units that could be anywhere in the world in a day or two, but were very lightly armed.

Heavy Armor units that were well outside the capabilities of most foes to match, but it could be weeks to a month to get them into theater.

The Marines on the other hand offered a fairly rapid (not paratrooper rapid, but a few days to a week depending on distance from Marine forces at sea to crisis spot, perhaps even hours if they were already postured), that could if need be show up with significantly more firepower than paratroopers (limited heavy armor, mechanized/motorized transport, light armored recon, organic fixed and rotary wing air support etc).

This speed and flexibility established the Marines as the short term crisis force of choice for the US military. They lack the numbers, firepower (keep in mind the US Army units at Fort Riley KS alone have more tanks than the entire USMC), and logistical "tail" to conduct extended operations alone (historically Marines rely heavily on both US Navy and US Army logistics for major operations), but deployed early and correctly they can stop, or at least slow down a crisis, or intervene early to build time and space for Army deployment.

So that would be the role that makes them distinct at least. Why this hasn't just become the "Marine" branch of the Army in modern times, I would point back to the public relations/engagement front. Rolling the Marines into the Army on a public relations front would be the same in many people's eyes to tearing down the Washington monument as a cost saving measure (there certainly remains some really good arguments for adjusting how forces/missions are spread over the US defense establishment).