[Suhiir]

I did use the word "intended".

Ever since Korea (1950-1953) the USMC has been used in sustained operations. While of course they can (and do) do so regularly it's not their "intended" mission.

Some of this is due to the US Army's over emphasis on mechanized warfare. From about 1970 thru the early 2000's the US Army seems to have almost totally forgotten about the need for leg infantry. Everything but their Rangers, Airborne, and the 10th Mountain was so tied to vehicles (helos in the case of the Air Cav) that without them they were basically non functional.

The USMC has always maintained that while infantry may occasionally ride/fly somewhere it's primarily a foot mobile organization.

Then you get something like Afghanistan where much of the terrain is mountains there's no option but to keep the Marines around while the Army reinvents "leg infantry".

During Vietnam the length of an individuals deployment was mandated from Washington, thus was the same for everyone (more or less). As you recall in Somalia the Marines went in first, then left (pretty much) when the Army arrived. The need for large numbers of troops in Iraq/Afghanistan means that given the size of the US Military overall there's no choice but to keep Marines there indefinitely.