They will serve as they always did, on ship, as the bodyguards of the officer class.
"Bodyguards of the Officer Class"?
In case you did not notice, this is 2013, not 1913. The Marines have not been involved on ships much since WWII.
The marines were not in the ETO but in the Pacific. The invasions in Europe including DDay were by army and the European pattern was an invasion then a long-long series of battles with divisions taking casualties over a long period of time.
The pattern in the Pacific was different, generally an invasion, then a short difficult battle and then preparation for the next invasion.
Many army divisions in Europe spent longer times in combat with many more casualties.
But the tempo was very different. The Army constantly had fresh troops and supplies come in to fill in the losses. They could also retreat if needed if a situation got to bad, and the combat was rather erratic for the most part. Move forward, fight, move forward, fight.
The Marines in the Pacific fought a very different war. For the most part when a unit takes 10% casualties, it is considered to be "combat ineffective" and pulled off the line fore replenishment. In many of the battles in the Pacific we saw Marine units with 30-50% casualties, and they continued to fight. And I can give one example in particular.
2nd Battalion 2nd Marine Regiment on Tarawa had horrendous casualties. Echo Company lost every single Officer in the first 10 minutes, and by the end of the battle was led by a Sergeant (E-5). The Battalion Commander was also killed in the opening minutes, and the XO was wounded in the first hours. At the end of the Battle a Captain was leading the entire Battalion. Casualties for this one battle were almost 50% for that Battalion.
And after being rotated back to the States and picking up fresh Marines out of infantry school, they were thrown right back into the grinder at Saipan, Tinian and Okinawa (the bloodiest battle of the entire war).
These are the kinds of examples that happened to only a few units in the Army (506 PIR), but this was the norm for every Battalion in the Corps. Try to watch Band of Brothers, then watch The Pacific, and try to tell me that they are even close. Especially when you realize you are talking about the most involved elite unit in the Army in the ETO, and in the other one you are seeing the experiences of average Marines.
For the Marines there was also no place to retreat to, their backs were always against the water. The Army fought like that a few times (Italy, Normandy), but that was about it. Every other operation had a way to withdrawal, even Market Garden.
No retreat was possible from Tarawa, Iwo Jima, Saipan, Okinawa, etc. It was go forward or die, no other option. Imagine that every battle the Army fought was like Normandy, that the entire series of Band of Brothers was like the opening minutes in Saving Private Ryan.
Red-Herring historical debate in meaningless.
The question is what will the Marine Corps do in the FUTURE?
WW II tactics are obsolete. Island hopping strategy would be ridiculous today.
Some of the military runs and depends on its image for survival, the marine corps is one such unit. The tactics the marines used in WWII are still the basic image most Americans have of that branch, but new specialized units keep coming along in competition. The latest new-comer might be the navy seals. I wonder if these specialized units do indeed compete?
Actually, while that may be the image you have, it is far from the truth. There is also Beirut, Haiti, and a great many others like Khe Sahn which spring immediately to my mind.
And it is not that "WWII Tactics are obsolete", we have simply not needed to do many amphibious landings in 60 years. However, remember that this legacy also kept Saddam totally off-guard during the Gulf War. He was so obsessed that the "Marines would land" that he had huge amounts of his forces along the shores, with their guns pointed to the East.
In a perfect position to get cornholed by the Army coming in from the West and South. The Marines then simply landed in Kuwait City - by Helicopter.
The Marines are a specialized branch, as are the 101 Air Assault, 82nd Airborne, and many other units. Basically in modern terminology the Corps would be considered to be Air Assault Infantry with Amphibious capabilities.
And trying to compare the Marines to SEALs is not even close. There are only around 2,000 SEALS, there are more Marines then that in a single Regiment. Seals are an ultra-light Special Operations force, like the Green Berets. They are not really Infantry at all, and in an actual 1 on 1 type unit engagement would actually probably have a hard time when you compare equipment and capabilities. They are to get in unseen, complete a mission and get the heck outta Dodge. Not dig in and try to hold down the fort until backup arrives.