I recall reading an article years ago where a senior Marine (4 star type one each) wrote that there was no real competition between the Army and the Marine Corps for mission because the Marine Corps is a battle winner while the Army is a war winner.
I took the general to mean that the Marine Corps is historically an assault force to be used primarily for short periods of very intense combat and then to be withdrawn to rest, regroup and refit. Given that operational reality, the Marine Corps has, shall we say, a certain “attitude” about how it postures its force structure, equips and trains those forces, and a very deep commitment to mission success regardless of casualties. (Well, at least it did while I was in and I’m pretty certain it still does.)
By contrast, the Army has to have the size, force structure, and in-depth logistics needed to sustain combat operations of widely varying intensity over long periods of time and it must supply forces for a multitude of highly varying missions in widespread locations.
There is certainly mission overlap (especially the part about closing with and destroying the enemy with fire and close combat) but the breadth of mission responsibilities is a lot larger than for the Marine Corps.
Once you shift focus in that way, a lot of difference in the size, composition, capacities, and institutional attitudes between the two forces makes sense.
You could assign Army missions to the Navy but you would soon find the Navy building another Army-like force to perform them. Or the Marine Corps would morph into something that, due to its sheer size and wide mission set, wasn’t quite the Marine Corps any more.
The first three incorporate the traditional tripartite service branches of Army, Navy, and Air Force, with the PLA Rocket Force dealing with conventional and strategic ballistic missiles, while the PLA Strategic Support Force seems to be a service directed at hi-tech and cyber warfare.
It seems that the PLA Rocket Force and PLA Strategic Support Force were established because the Chinese leadership wanted new, specialized services run by people they controlled and had confidence in. More fundamentally, different arenas of conflict inevitably call for different commands and suitable weapons, skills, and specialties.
Very good posting about the differences between the USA and USMC.