system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why the US needs a Marine Corps when the other three services could potentially fulfill its role.

Other than the National Security Act of 1947, what is the legitimate reasoning behind maintaining what some would deem to be a second standing army?
gpt
flexibility. no one is pretending that in terms of land combat the Marine Corps is anywhere as close as capable as the Army is, to illustrate this, it's composed of three active infantry divisions with supporting arms (and of course the air wing), the Army has (multiple?) divisions *completely composed of armor*. the Marine Corps has several (like, 3) *battalions* of tanks. suffice to say, Marines weren't guarding the fulda gap in the cold war, nor were they ever intended to.

that being said, the Marine Corps possesses every warfighting capability of the United States armed forces, at some level, organically. A Marine commander, possibly as junior as a LtCol (and they are certainly trained as such) could have under his direct command everything from an infantry regiment with supporting arms, including arty and tanks, with requisite logistic support, to limited heavy airlift capability (C130s), air superiority (F/A-18s), and even electronic warfare (prowlers).

so this brings us to its point in national defense strategy. if something happens "flash in the pan" a Marine commander can have enough assets at his fingertips, instantly, to bring a lot of pain for about two weeks to maybe a month with almost zero outside assistance. obviously this isn't being applied in protracted occupations/COIN like Afghanistan (or to be honest even OIF-1), so this capability, *literally* the primary purpose of the Marine Corps, isn't really seen.

but this cohesiveness and integration at such a comparatively low level is exactly the purpose of the Marine Corps. every asset in the Corps (except of course the grunts themselves) is reminded that they exist solely and utterly to support the rifleman on the ground. training, for every MOS, is centered around an expeditionary capability to allow this. MEUs are a regular deployment that are a specific embodiment of this philosophy: a purpose-built small deck carrier with an infantry battalion, some supporting arms, a logistics arm, and a composite aviation element of helos, ospreys, and harriers, with 130s on call. the CO (a colonel) has all this at his command as it sails around its assigned hemisphere, waiting for something bad to happen.

tl;dr while much smaller, the Marine Corps is structured to allow to be *entirely self sufficient* (barring Naval assets like LHDs, who themselves exist solely to support the Marine Corps) for a **short period of time**

edit: OP you're clearly a Marine, don't you know all this shit?

gold for this? sweet, thanks!