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Of course, infantry carried on transport ships and destroyers can be instructed and thrown to battle, with high likelihood of been massacred in any opposed landing. US was not that desperate in 1940, am i right?
Well, I don't know where this TL is going so I won't say there wont be a emergency, or perception of emergency.
I think the 1st Provisional Marine Brigade was untouchable at moment (losing it mean losing all the tutors in amphibious warfare), and 2nd Marine brigade in Pacific contained in October 1940 only 8th marine regiment, formed in April 1940 (and containing only 2 battalions in the moment). These 2 battalions were the only force US had in the moment in Pacific to try to counter-attack on ground. For the rest of fighting, US forces in Philippines and Guam had to rely on naval interception and static defenses. ...
These Brigades were not set infantry organizations like the brigades of most armies of the era. The were supposed to be task organized combined arms organizations. The brigade staff trained for operations as part of a naval task group. The brigades nominally included a light artillery battalion, a armored company, engineer group, automotive transport unit, signals unit, and a composite air group, all in addition to the rifle regiment. In theory a base defense battalion could be included in the brigade if needed, tho I don't know if that was ever done in any training exercises. These brigades pretty much reflected the TF nature of the corps size amphibious forces deployed in the Pacific during the 1940s. Reduced size models in a sense. The deployment of a brigade to Shanghai in 1927 & the standing up of a combined arms group in Nicaragua for several years was done by mix and matching components. Or creating them as required. ie: The pack mule herd used by the Quartermaster company in Nicaragua.
The s acrosancisity of the east coast brigade had to do with the perception of priority within the Navy. The developing war in the Atlantic and necessity to prepare for operations there meant the Marine units on the east coast, in or outside the brigade were strengthened at the expense of the west coast units. If the US is actually at war with Japan the priority is going to shift the other direction.
I may have missed the status of the USN reservists in the previous TL posts. The War Powers Acts passed after France collapsed allowed the activation of the entire USN reservist component, including the Marines. That activation was a assumption within WP ORANGE so its likely to occur swiftly if a Pacific war occurs.
Bottom line here is the Navy was in much better shape in its small USMC component being combat ready, than the US Army & its field forces. That probably had more to do with Congressional budget decisions since 1929 than any thing else. Funds for training and readiness were effectively eliminated for the infantry, artillery, and cavalry corps in the 1930s.