“Not only do Marines fight, they fight far more often and in far more places than any other branch.”

~ Anonymous

United States Marine Corps Organizational History

How are Divisions, Regiments, and Battalions structured to make up the United States Marine Corps in its entirety? This segment addresses only infantry/artillery units and not air, logistical, or support elements. This information is intended to show the simplicity of Marine Corps unit structure with simple numbering. The United States Marine Corps is a department within the Department of the Navy, which is headed by the Secretary of the Navy (SECNAV). The Commandant of the Marine Corps is responsible for the entire USMC organization, recruiting, training, and equipping of all units so that it is ready for operation on a moment’s notice.

The Table of Organization (T/O) of the Marine Corps lays out the organizational structure and the equipment required to function. The T/O started out as a chart-like document published by the War Department during WWII, prescribing the organic structure and equipment of units from divisional strength on down to the smallest operational entities, i.e., Platoons, Squads, and Fire Teams. Prior to 1943, organization and equipment were in Tables of Organization (T/Os) and Tables of Basic Allowances (T/BAs). Beginning in August of 1943, to ensure coordination between organization and equipment, a consolidated Table of Organization & Equipment (T/O&E) was instituted for each standard unit throughout the military.

Let us use 1/27 because it is the unit of discussion in this history. It is referred to amongst Marines as a member Battalion of the 27th Marine Regiment. The 27th Marine Infantry Regiment (commanded by a Colonel) when activated consisted of a Headquarters and Service Company and three rifle infantry battalions, the First, Second, and Third Battalions. With three Regiments in a Division, accordingly, the 26th, 27th, and 28th Marine Regiments reported operationally and administratively to the 5th Marine Division (Commanded by a Brigadier General). All Marine Regiments are structured in the same modality. To simplify the naming of all Marine Corps Regiments, they were given numerals with the addition of Marines: thus 1/27 aka First Battalion, 27th Marines, meaning the First Battalion of the 27th Marine Regiment; 2/26 for Second Battalion, 26th Marines or 26th Marine Regiment, and so on.

To give the reader an overview of the United States Marine Corps in its entirety and full personnel potential (Infantry & Artillery only), this following rudimentary T/O (Table of Organization) example from World War II displays Division and Regimental makeup at full capacity:

1st Division: 1st, 5th and 7th Marines (infantry); 11th Marines (artillery)
2d Division: 2nd, 6th and 8th Marines (infantry); 10th Marines (artillery)
3d Division: 3rd, 9th and 21st Marines (infantry); 12th Marines (artillery)
4th Division: 23rd, 24th and 25th Marines (infantry); 14th Marines (artillery)
5th Division: 26th, 27th and 28th Marines (infantry); 13th Marines (artillery)
6th Division: 4th, 22nd, and 29th Marines (infantry); 15th Marines (artillery)

Both the 5th and 6th Divisions were decommissioned after WWII and the 4th Division evolved into our Marine Corps Reserve units to this present day.

The 4th, 22nd, and 29th Marine Infantry Regiments were already combat-proven units and became subordinate units of the 6th Marine Division assembled in September 1944. The 15th Marines, artillery regiment was also activated and thus placed in the 6th Marine Division. The Sixth Marine Division never set foot in the United States—being formed in the Pacific, fought in the Pacific, and disbanded (colors retired) there.

When assembled, the Sixth Division was a grouping of already battle-hardened regiments who had been tested back to the beginning of the war in the Pacific. The 4th Marines were the Raider Battalions; 22nd Marines fought on Eniwetok; the 29th Marines had been on Saipan. They were independent roving units in the past, now chosen for upcoming Pacific battles. Major General Lemuel C. Shepherd, Jr., whose experience went all the way back to WWI, was their leader. Along with the 5th Marine Division, they were destined for historical greatness toward the end of the war in the Pacific. The 27th Marine Regiment was formed in January of 1944 and saw the first action on Iwo Jima in the Pacific as a Regiment of the 5th Marine Division. The “Easy” Company Marines of 2/28 of the 5th Division were responsible for raising the flag on Iwo Jima twice. After the war, the entire 5th Marine Division was deactivated in 1946.

The 26th, 27th, and the 28th Regiments of the 5th Division were reactivated again during the Vietnam War in 1966. The 26th Regiment, sent to Vietnam in March of 1966, played a prominent role in the northern I Corps area of the Republic of South Vietnam. It was attached to both the 1st and 3rd Marine Divisions at various times during its deployment. The 27th and 28th Regiments trained personnel as replacements for the units already in Vietnam. The 27th Marine Regiment was ordered to Vietnam in early February of 1968 by Lieutenant General Victor H. “Brute” Krulak, Commanding General, Fleet Marine Force-Pacific (FMF-PAC) when he received notification through the chain of command to send additional Marines. The 28th Marine Regiment remained in California as the Division reserve and training element.

The above listings of Divisions and their Regiments identify for the reader a mental arrangement of the basic Marine Corp’s infantry and artillery structure of the Division’s T/O when it was at its zenith during WWII. During Vietnam personnel strength and Regiments functioned differently. The new acronyms OPCON (Operationally Controlled) and ADCON (Administratively Controlled) came into use to maneuver the units throughout I CTZ in Vietnam. The 1st and 3rd Marine Divisions were assigned to the combat zone of the I CTZ while the 2nd Division remained on the east coast, and whose missions focused toward the Atlantic and eastward. The 4th Marine Division was the Reserves.

The USMC Table of Organization and Equipment in present-day operates with only the first four Divisions. The 4th Division functions as the home of the United States Marine Corps Reserve. The 5th and 6th Divisions exist only on paper and in history with their colors in retirement at Marine Corps Headquarters.

Each Division during the Vietnam War represented a combat force on average of roughly 22,000 – 25,000 personnel and fluctuated in troop strength monthly due to tour rotations, KIAs, and WIAs. Of course, there were support organizations such as Engineers, Logistics, Air-Ground Support, and Intel. They were and are indeed extremely important organizations linked to the success of the Marine Corps mission but are not shown in this overview.