system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What purpose does the USMC serve that the army doesn't?



It's not like they only use the navy to get around, and the army deployed by sea at normandy.

Why don't I ever hear about marines fighting in europe or the army fighting in the pacific? (WW2) What was different about their combat doctrine at that time that determined who got what job?

What do they actually do differently now?

I don't think the marines sailed to Afghanistan; how are they different from the army?
gpt
The idea is that the USMC can serve as a fast reaction, forward deployed force. The army isnt neccesarily set up to have forces of various, but usually smaller size forward deployed at bases around the world. How have MAGTF's (Marine Air Ground Task Forces) of various sizes, at the smallest it will be an infantry battalion, along with some armor, some logistical, and some air support, which you can all load onto a couple of USN ships and sail around until needed.

The USMC is set up both structurally(all branches within one, and strong cross training with USN), and doctrine wise to do that (focus on warrior ethos/smaller more multi task units), to go into situations with minimal outside support, and to make do. The Army meanwhile is the big shock and awe force which comes with everything you can imagine and then some. The USMC is then the smaller faster sword, to the USA's giant 105mm howitzer.