I'd like to thank Gordon Rottman for providing permission to post this article.
On 8 Sep 39, the President declared a "limited emergency" due to the threat of war in Europe and this began a general expansion of the Marine Corps. On 15 Sep, the General Board of the Navy completed an assessment of the Navy's readiness, the day before the invasion of Poland. It concluded that the US Fleet did not possess the capability to seize or defend Western Pacific bases and that developed US bases were deficient in numbers and capability. This was important to the USMC in that one of its main missions was (and is) to seize and defend over seas naval operating bases. The report also found that the Navy lacked sufficient transport and landing craft to support the Fleet Marine Force (FMF), a force severally short of modern weapons and equipment. The conduct of amphibious operations was the third priority 3after battle fleet operations and the defense of existing bases.
The FMF is the USMC's operating force and contains all ground and air combat units as well as certain mobile support organizations. In Sep 39, had a total strength of just over 18,000 personnel; equaling the size of the New York City Police Department. Even though comparatively small, it was the world's largest marine corps, which the Marines liked to point out, was maintained at a cost of $1,500 per Marine as opposed to the 18th largest army sustained by the US Army at $2,000 per soldier. The FMF then consisted of:
1st Marine Brigade, FMF Quantico, VA
1st Marine Aircraft Group
2d Marine Brigade, FMF San Diego, CA
HQ Squadron 2
Marine Forces in North China
The USMC Organized Reserve consisted of the 1st-19th Marine Reserve Battalions scattered about the country in major cities. These were basically infantry manpower pools (the 7th was armed with 75mm guns). When mobilized in late 1940, they were absorbed into newly forming infantry regiments. USMC Aviation Reserve had ten scouting squadrons (1-10) and two service squadrons (2 and 3). I have not dug into what models and numbers of aircraft were assigned to which units.
There were a number of Marine Barracks (its a unit of 1-3 small companies, not just a building) and smaller Marine Detachments responsible for guarding navy bases, depots, and other facilities. They were not part of the FMF, but were under navy district commanders. Companies could be withdrawn to augment deploying infantry battalions. Battleships, carriers, and cruisers each had a company-size Marine ship's guard detachment, which were part of the Fleet.
During 1940, the brigades began to gradually expand with the regiments enlarged to three battalions, the engineer and medical companies expanded to small battalions, a two-battalion artillery regiment formed, the tank companies were converted to scout and a new tank company added. The antiaircraft battalions (redesignated the 1st and 2d Battalion, 15th Marines) were made separate, redesignated and reorganized as the 3d and 1st Defense Battalions, respectively, with the brigades keeping an antiaircraft company. The aircraft groups were withdrawn and Marine aviation began a general expansion. On 1 Feb 41, the 1st and 2d Marine Brigades were redesignated the 1st and 2d Marine Divisions, respectively.