I don't believe that there is any necessary difference between the functions and roles of members of the Army and members of the Marine Corps. Both groups, certainly, may be deployed in an assault role; it may be true that the Army requires a wider range of duties of its troops than the Corps (e.g., occupation duties after the most intense combat is over).

I'm pretty sure that the basis for the assertion "I'm a Marine, don't call me a soldier" (and vice versa) is purely semantic, and an important part of maintaining a group identity and esprit de corps. Not unlike "Don't call me 'sir'; I'm a sergeant, not an officer ~ I work for a living." In another nation, another culture, I'm sure it would be entirely proper to call both groups of warriors "soldiers."

Back to General Mattis: It seems disingenuous for people to get so up in arms about this guy's pronouncement. What the hell do they think war is all about? I don't necessarily like what he said, but I can imagine what it must be like to be thrown into a kill-or-be-killed situation, and the general's attitude is entirely appropriate and very much to be expected.

For the record: I am an avowed pacifist, but I served ~ reluctantly, and only after repeated "invitations" over a period of several years ~ as a draftee in the US Army, pulling noncombatant duty while petitioning for discharge as a consciencious objector. I managed to keep myself out of combat, but I came to know many men who had been there. So I have knowledge of, and a degree of sympathy with, a wide range of positions on issues of warfare.