The Marine corps as it developed in the 20th Century was a large departure from the 18th & 19th Century Corps. Earlier it had followed the Britsh model where the marines were relatively small and a integrated part of the ships company. Battalions of any size were temporary ad hoc formations and usually filled out with common seamen, gunners, and officers from the ships companies. For littoral & amphibious ops the US Army doctrinally and in real terms provided the bulk of the infantry, artillery, and other specialties for landing forces. After 1900 this gradually began to change. The Navy began looking at permanent 'battalions' of Marines, and deployable base defense units. In the early Banana Wars HQ for nominal Marine regiments were established to administrate the growing forces ashore in places like Hati. The occupation of Vera Cruz was a indicator of the trend. The US Army had no readily deployable formation & the seizure was executed with a mixed bag of Marines and seamen from the Fleets ships companies. Eventually the Army did get a brigade together to relieve the Navy landing party, but it was over a month.daveshoup2MD wrote: ↑04 Apr 2021 18:42The 1st and 2nd brigades of the FMF were organized as such in 1935-36, and included a headquarters, infantry regiment (3 battalions), field and AA battalions/batteries, engineer battalion, tank company, medical battalion, signals, service and support, etc., with dedicated aviation (at least nominally). These two formations were expanded into the 1st and 2nd Marine divisions in 1941, as part of the prewar mobilization, and provided the basis for the FMF's expansion during WW II to an army-level expeditionary force of two corps and six divisions, in total.
The Marines had brigade-level experience going back to W I (and before, actually), and there were enough Marines in France by 1918 that if the war had continued into 1919, there could have been a Marine Division in the field with the AEF.
Post 1918 then Navy began further development of the Marines as a ready force. The Base Defense Battalions continued to develop & extensive thought was put into what would be needed for War Plan ORANGE. This was influenced by requirements in the expanding Latin American interventions & possible execution of full war plans in Latin American nations, beyond Nicaragua. In Nicaragua you can see the outline of the Marine Expeditionary Brigade in actual combat operations 1926-1934. Another example would be the group deployed to Shanghai in the 1927 emergency. War Plan ORANGE still envisioned the US Army providing 80 - 90 % of the ground combat forces. But unlike earlier wars the Marines had become a formal component of the Navys vision of landing and ground ops in a Pacific War. The Expeditionary Brigades were permanent and sophisticated (even by modern standards) combined arms groups.
Even today the two are a lot more integrated than most people understand. The core role as a littoral combat force means the Marines must be closely integrated as a Navy component. Had the pre 20th Century doctrine continued & the US Army provided the men & arms for all landing forces the specific would need to be as closely integrated into the Navies amphibious forces in order to provide a ready ground combat component of a littoral expeditionary force.The USN provided (and continues to provide) medical services and (general) engineering support, via the USN's CEC - the Seabees.
The four air wings made that 300,000 something more than a field army. Unlike the near universal trend in the 20th Century the USN went the opposite direction and kept a closely integrated tactical air force with its Marines. Im unsure what mavericks in the Navy made that happen, but it exists.Which, it's worth mentioning, numbered 300,000 officers and men at the high point in WW II, so the equivalent (roughly) of a field army in manpower.