The United States Marine Corps is part of the US Navy, despite the Marines' fervent claims to the contrary, and despite the fact their paychecks clearly state "Department of the Navy", not "USMC".
Marines serve two functions: 1) Catching bullets 2)Poking mines with a stick. When there is a battle where heavy casualties are expected, the Marines are sent in... to be grist for the mill.
Guy 2: Oh, you're in the Navy.
Guy 1: What? Fuck you! I'm no swabbie fag! I'm a goddamn Marine!
Guy 2: I know. The Marines are part of the Navy, didn't you know that?
MARINE BY GOD!!! OORAH!!!
They only have 2 jobs to do;
1) Take the land our our enemies, and
2) Kill anybody who try to stop them from doing job #1.