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World War II represented a watershed in the development and employment of amphibious tactics for the United States. As the Marine Corps contemplated and planned for its role in future wars during the 1930s, it became clear that air power and armored forces would have a major impact in the way that Marine units fought in combat. Prior to the establishment of the First and Second Marine Divisions in February 1941, no unit larger than brigade-size had ever been formed in the Marine Corps. As planners envisioned what assets a division of Marines would need in war, it became clear that organic antiaircraft artillery and antiarmor weapons would form key parts of the divisional firepower. As the world inched closer to total war starting with events in Europe during 1939, Marines watched and studied the campaigns in Europe.
The original organization of the battalion under the D-100 T/O gave it a total strength of 856 officers and enlisted personnel. It looked like this: Headquarters Battery with 85 Marines and 14 Naval medical personnel,
In April 1943 the Marine division was reorganized under the E-100 Table of Organization based upon campaign experience in the South Pacific. Among the many changes to the division, the Special Weapons Battalion was modified to a strength of 757 officers and enlisted personnel. It looked like this: Headquarters Battery with 88 Marines and 14 Naval medical personnel, Of specific note in this T/O was the deletion of the 90mm AAA Battery from the battalion structure. Only the first three Marine divisions had these batteries, and the guns and their crews were transferred to newly forming defense battalions.
In the third major divisional reorganization of the war, the F-100 Table of Organization deleted the Special Weapons Battalion. The 75mm Gun Motor Carriages and their crews were assigned to the regimental weapons companies already in the divisional structure. This increased the number of 75mm GMCs in each weapons company from two to four guns. The 40mm AA batteries were assigned to newly forming antiaircraft battalions. The battalion's 37mm antitank guns were shipped to depots as spares since the divisional structure already had 36 of these weapons in the regimental weapons companies. Table of the Special Weapons Battalions and their Campaign Participation
ANTIAIRCRAFT DEFENSE Two key weapon systems were identified as being essential to the divisional air defense structure. They were the 90mm M1A1 Antiaircraft Artillery Gun, and the M1 40mm Automatic Gun. The 90mm AAA gun was designed to provide high-altitude air defense, and the 40mm had the mission of mid- to low altitude defense. It was planned to interlock these weapons in the divisional air defense plan. They were to be supplemented by the firepower of the .50 caliber machine guns in the division for low altitude antiair protection. The Marine Corps originally intended to use 20mm antiaircraft guns to combat the low altitude air threat, but these weapons were never procured for divisional units, and .50 caliber machine guns took their place in the inventory. Doctrinally, antiaircraft assets were landed as early as possible in the amphibious assault to build an air defense system for the landing forces. In particular during the campaigns up the ladder of the Solomons, the Japanese and American air forces maintained a rough parity of strength, which meant that ground forces had to be prepared to defend themselves against air attacks. On Guadalcanal, for example, Japanese bombers raided the American airbase at Henderson Field many times in the fall of 1942. AA guns were linked together in a system to provide maximum coverage across the Marine perimeter. The 90mm and 40mm Batteries of the 1st Special Weapons Battalion were attached to the 3rd Defense Battalion on Guadalcanal and maintained air defense as part of the overall defensive scheme on the island.
As the primary high altitude antiaircraft gun of the Marine antiaircraft artillery, the 90mm M1A1 gun equipped not only the divisional special weapons battalions, but also the AAA batteries of Marine defense battalions. Later in the war, these weapons also equipped the batteries of AAA battalions. The 90mm AAA gun replaced the 3-inch M3 antiaircraft gun, which was the original equipment issued to Marine antiaircraft units. Development work on the M1A1 began in 1938 when the US Army recognized the need for an heavy antiaircraft gun able to engage rapidly maneuvering high altitude bombers. Work continued on the gun system through January 1943, when the final World War II-era models were type-classified.
The 90mm gun section was commanded by a sergeant, designated as the section chief. A corporal served as the gunner, and also commanded the gun squad. One Marine was responsible for traversing the gun, and another for elevating and depressing it. A corporal and four Marines were assigned to the ammunition squad. Finally, a driver was assigned to the section. The gun was towed by a 2 & 1/2 ton truck, or sometimes by a bulldozer for short distances. The battery was usually aimed and fired at as a coordinated unit against aerial targets. Battery headquarters was connected by field phones to the guns, and the fire direction center telephoned deflection, elevation, and time of flight out to the gun sections. The headquarters' director squad gathered data on the incoming targets's direction, altitude and speed, and then fed this data into the director set, which was a simple computer that estimated elevation and deflection for the guns, as well as flight time in seconds to the target. Information on the enemy's location usually came via radar, typically the SCR-268 search radar. In an emergency, the direction squad could could the enemy's speed, range and altitude by visual estimation. Upon receiving the data, crews set their guns, and fired at the coordinates, usually in massed fire at a single target. During the 1942-1943 timeframe, the Marine Corps utilized the M5 Director Set, although this was later replaced by an improved model.
40MM M1 AUTOMATIC ANTIAIRCRAFT GUN The 40mm AA gun served as the backbone of the divisional medium altitude air defense system. As a dual purpose weapon, the 40mm could be employed against both air and ground targets. In the 1930s, Bofors of Sweden developed the 40mm AA gun, which was standardized in the US Army and US Marine Corps in July 1942. In the United States, these weapons were manufactured by Chrysler, Blaw-Knox, and York Safe and Lock.
The 40mm crew was designated as a section, commanded by a sergeant. The gunner was a corporal, and also led the gun squad. Two gun pointers controlled traverse and elevation of the gun, and the loader/firer fed and fired the 40mm rounds. Two ammunition handlers passed loaded clips to the loader/firer. In addition to the 40mm gun, the section had a .50 caliber water-cooled heavy AA machine gun assigned as part of the gun squad. A gunner and loader served this weapon, which provided low altitude air defense for the section area of responsibility. The ranging squad was led by the range setter, a corporal. There were two elevation trackers, one for azimuth, and one for elevation, who also acted as lookout/spotters. In addition the squad had a generator operator, and a field wireman. One of these Marines also acted as the driver, and the section had a prime mover, usually a 2 & 1/2 ton truck. The 40mm could be fired either optically using the gun sights, or by director control using the M5 Director Set. This was the same device used to control fires of the 90mm AAA gun. The diagram below depicts the fire control system of the 40mm gun section.
ANTIARMOR DEFENSE
M3A1 37MM ANTITANK GUN
There is a complete entry on the 37mm antitank gun in the World War II Gyrene weapons and equipment section. This entry includes full information on the weapon's development, employment, and gunnery.
M3 75mm GUN MOTOR CARRIAGE
There is a complete entry on the M3 75mm Gun Motor Carriage in the World War II Gyrene weapons and equipment section. This entry includes full information on the weapon's development, employment, and gunnery.
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