The only reason the USMC exists is to be the Navy's Infantry. Hence, their name.
If an Army General and a Navy Admiral are taking part in an Amphibious landing, and the Navy has no Infantry, the Army is in charge. The Army is the Senior Service.
But Army Generals trying to tell the Navy what to do doesn't work very well most of the time. There have been multiple examples in multiple Wars. Catastrophic examples
So we gave the Navy their own Infantry to keep the Army from sinking Navy Ships. bummer. We call them 'Marines' and their job description comes down to, "Hey Diddle-Diddle, Charge Up The Middle". And they're some of the best, most fearless light infantry in the world.
But that's what they are -- "Light" Infantry. They get a Beachhead, and the Army comes in and takes over. That's the way it's supposed to work. And that's what the Pentagon wants to get back to.
It can get complicated but I agree with it. No knock on the Marines. They seize and hold the Beach until relieved by the Army.
If we could just keep them from eating the crayons........