More recently the marines have been taking on the role of light infantry instead of naval infantry. That’s fine, there is a fair amount of overlap, but if they want to be able to operate in the Naval infantry role, submarine TTPs are needed.

Regarding ship boarding: war breaks out, a supertanker is preparing to transit the Malacca straits. Submarine closes to within torpedo (or missile) range. Orders tanker to stand to. Submarine gets close to board, depth charges set by foreign agents are released (war probably will be anticipated by the other side by at least a few weeks). Alternately a handful of marines board a small boat or three and takes control of the ship with a lot less risk. Alternately, a 8-10 hr wait for a SEAL team to arrive — if they aren’t needed somewhere else. Alternately the submarine escorts the tanker out of the war zone, taking itself out of action. A problem that our Navy has is that a lot of its assets are too expensive or irreplaceable to risk. That includes a SEAL team IMO. This is where the Marines would come in.

The gradual spread of anti-ship and anti-aircraft missiles means that the era of amphibious operations is coming to an end. There is no conceivable scenario in which the US could put troops ashore against even a second-tier country like North Korea. For reasons unknown, Hussein twice permitted hostile buildups when he could have crushed them before significant forces were landed. No one will ever permit that again.

The only way we can put significant forces ashore anywhere in Eurasia is if we have a sanctuary far from the front, meaning France. Large-scale land warfare between major powers might be impossible.