After the Spanish-American War, and prior to the First World War, the Marine Corps was kept at a size of approximately 17,000. The Corps was a strictly volunteer only force, and did not accept conscription when it was implemented during the Great War, but the Corps did rise, by November 11th, 1918, to a force of nearly 70,000 men, many of whom saw significant combat, particularly at Belleau Wood, a battle that many historians regard as the engagement that made the Marine Corps internationally famous.
After the end of the First World War, the United States began an immediate draw-down of its armed forces to peacetime levels. The size of the US Armed Forces was further reduced by the Defense Act of 1921, by which time the Marine Corps was once again restricted to no more than 17,000 men.
Although its numbers were reduced and its funding restricted, the Corps was able to graduate about 125 to 150 officer candidates via the United States Naval Academy at Annapolis each year by 1933. Further officers and NCOs were "poached" from a rather unexpected place - the US Army ROTC. Since the Army was also under size restrictions and had a limited budget for officers in peacetime, the Marines were able to recruit the best and brightest Army ROTC candidates by offering employment and a highly likely chance of action in the field: a fast track to career advancement. This was a remarkable feat, considering that the United States Military was described by one period source as (sic) "...one of the most depressing career paths a young man can choose to follow."
By 1933, the USMC was around 15,000 personnel strong, with the majority serving in active duty positions and split between the East Coast and the West Coast. Traditionally, East Coast Marine units were deployed on operations to the Caribbean, Central America, South America, and Africa, while West Coast Marine units were used almost exclusively on Pacific duty stations and, increasingly since the end of the Great War, in China.
It has been estimated that of the 15,000 Marines serving at the time of the outbreak of hostilities, about 70% remained loyal to Washington and to the Nationalist Government or were actively deployed abroad, as in China, or in the US Pacific Territories, such as Hawaii, and played no role in the Early War fighting. About 30% chose to align themselves with the Constitutionalist movement. The majority of Marines choosing to side with the Opposition Government were stationed on the West Coast, but there were exceptions to the rule, and the same can be said of the Marines deployed on the East Coast.
Most of the armored vehicles in Marine Corps service (consisting, at the beginning of the war, of six M1917 Six Tons in inactive reserve and approximately six King Armored Cars) remained in Nationalist hands, but two King Armored Cars stationed near San Diego did fall under the control of a Constitutionalist Marine unit fairly early on. Of course, the sixteen Disston Tractor Tanks (classified by the Marines as the M1933 Disston) ordered prior to the outbreak of hostilities in 1933 were delivered to Quantico upon completion and began their service in Nationalist hands.