How did the British Army unit size and structure in WWI change from 1915 to 1918?

Question

*Preface: For obvious reasons, we will assume ideal conditions for the purposes of this question. For convenience's sake, we will also ignore the original British Expeditionary Force that was almost wiped out by early 1915, and instead, limit the scope of the question to the "New Army" (AKA "Kitchener's Army") that took to the field from mid- 1915 onwards. And I know that some units were unique, sometimes drastically different from the norm, so answers will, of necessity, be general and not universally applicable.* I'm curious as to how the size and structure of British Army changed from 1915 to 1918. As I understand it, in 1914 the theoretical ideal was as follows: ##Division: Roughly 18,000 men, including 4 infantry brigades plus other units like engineers, medical personnel, artillerymen, signallers, HQ staff, etc) ##Brigade: Roughly 4,000 men, in 4 infantry battalions plus other units ##Battalion: Roughly 1,000 men, in 3 companies plus other units ##Company: Roughly 230 men, in 4 platoons plus other units ##Platoon: Roughly 50 men This changed over time - the cavalry was drastically reduced, stokes mortar teams were added, more machine gun teams were created, etc. #Starting in 1915, excluding special cases, and assuming ideal conditions, how did these numbers change - including the number of men per unit (e.g., ~1,000 per battalion) and the number of subsidiary units per larger formation (e.g., 4 battalions per brigade)?

Answer 1

There are two important corrections to the starting organization in the question. A standard British infantry division in 1914–15 normally had 3 infantry brigades, each containing 4 battalions, giving 12 infantry battalions per division. A battalion normally contained 4 companies, each divided into 4 platoons.

At full 1914 establishment a battalion was approximately 1,000 all ranks (1,007 is a commonly quoted establishment). Thus a simplified 1915 New Army structure was approximately:

Division

1. about 18,000 men overall
2. 3 infantry brigades
3. 12 infantry battalions total
4. plus artillery, engineers, signals, medical, transport, headquarters, etc.

Brigade

1. 4 infantry battalions
2. roughly 4,000 infantrymen before allowing for the brigade's other personnel

Battalion

1. approximately 1,000 men at full establishment
2. 4 companies

Company

1. 4 platoons
2. approximately 200–250 men at establishment

Platoon

1. roughly 40–60 men, depending upon the particular establishment and period

The important story from 1915 to 1918, however, is not simply that these formations became smaller. Their firepower and specialization increased enormously.

In 1915 the battalion was still predominantly a rifle formation. Heavy machine guns were progressively removed from the individual infantry battalions and concentrated in the newly created Machine Gun Corps. At the same time, Lewis light machine guns began entering infantry service. The initial establishment was only a few Lewis guns per battalion, but this increased dramatically during the war.

By 1917 the platoon itself had become a miniature combined-arms organization. A typical platoon was organized around four specialized sections:

1. riflemen,
2. bombers using hand grenades,
3. rifle-grenadiers, and
4. a Lewis-gun section.

Thus the British Army increasingly substituted firepower and tactical specialization for masses of riflemen. The platoon rather than merely the battalion became capable of generating several different types of fire.

The largest organizational change came in early 1918, partly because of the British Army's manpower shortage. Infantry brigades in France were generally reduced from 4 battalions to 3.

Therefore the normal divisional infantry organization changed from:

3 brigades×4 battalions=12 battalions3\text{ brigades}\times4\text{ battalions} =12\text{ battalions}

to

3 brigades×3 battalions=9 battalions.3\text{ brigades}\times3\text{ battalions} =9\text{ battalions}.

Numerous battalions were consequently disbanded and their men transferred as reinforcements to surviving units.

Meanwhile, machine-gun organization became increasingly centralized. By 1918 the machine-gun companies associated with the division were consolidated into a divisional Machine Gun Battalion.

At battalion level, however, light automatic firepower had become much greater. By July 1918 an infantry battalion's establishment included 36 Lewis guns. The late-war platoon was reorganized again, with greater interchangeability among soldiers rather than the rigid specialist organization of 1917.

So, in very simplified form:

1915:

Division → 3 brigades → 4 battalions each → 4 companies each → 4 platoons each.

1918:

Division → 3 brigades → 3 battalions each → 4 companies each → 4 platoons each.

The apparent reduction from 12 to 9 infantry battalions per division should therefore not be interpreted as simply making the division weaker. The 1918 infantry battalion had vastly greater automatic weapons, grenades, rifle grenades, mortars, improved artillery coordination, signals, and tactical specialization than its 1915 predecessor.

One final distinction is important: establishment strength and actual trench strength were very different things. A battalion theoretically established at hundreds of men could be drastically understrength after sustained fighting. Therefore, when comparing 1915 and 1918, it is best to compare official establishments rather than the actual headcounts of individual battalions after battles.