Britain has a very long history of organising local, voluntary bodies for the army. Significant reforms took place in 1908 which organised the local volunteers into the Territorial Force. In terms of its organisation, equipment and training doctrines, it followed closely the models of the Regular Army. However, the TF was composed of volunteers, men who would train as soldiers on a part-time basis - hence the nickname of Saturday Night Soldiers. These men had no obligation to serve overseas (although when asked the vast majority agreed to do so). This factor may have been part of the logic applied by Kitchener when he saw the need for huge expansion of the army for continental land war: instead of building on the existing local infrastructure for recruitment, administration and support already provided by the TF, he instead created the New Army. The eventual expansion of the TF was by duplicating the existing units into the so-called Second and Third Line formations.

Battalions of TF infantry were organised along the same modern lines as the professional army, together with higher formations and all of the necessary support units (transport, medical, signals, engineers, fortress garrisons and so on). Similar organisation of the volunteer artillery and cavalry on regular Army lines also took place in 1908. The infantry battalions were aligned to the local county regiments (for example the volunteers of the Wolverhampton and Walsall areas became the 5th and 6th Battalions (TF) of the South Staffordshire Regiment.)

The County Regiments had up to four battalions of Territorials. This did not apply to the Guards or Rifle Regiments, nor to the Regiments from Ireland.

The principal role envisaged for the TF in time of emergency was home defence. The 'Terriers' were not obliged to serve overseas in the event of a crisis.

Men who joined the territorials came from all backgrounds, and gave up a night or two a week to train. They also went on an annual camp which for many was a highlight indeed, as many otherwise rarely travelled beyond their home town or county.

In all, the infantry was organised into 14 Divisions of the Territorial Force. In 1914, each followed the regular army pattern of having three infantry brigades, each of four battalions. The other Divisional troops consisted of four Brigades of artillery, one Battery of heavy artillery, two Field Companies of territorial Royal Engineers, one Signals Company, a Divisional Train (supply and transport column), three Field Ambulances of the territorial Royal Army Medical Corps, and a Clearing Hospital.

The enthusiasm of the Terriers for the war in the early days was measured by the fact that the majority signed up for overseas service. This enabled units to be mobilised and sent overseas - initially, with some battalions being sent to the Western Front. Soon, entire TF Divisions proceeded overseas, either to the fighting fronts or as replacement garrisons as the regular troops were deployed to action. Many of the Terrier units soon established strong reputations, and the early doubts about their value were removed.

On 15 August 1914, orders were issued to separate the 'home service' men from those who had undertook to serve overseas, with the intention of forming reserves made up of those who had not so volunteered. On 31 August, authority was given to establish a 2nd-Line Division for each of the 1st-Line where more than 60% of the men had volunteered. These Divisions were formed from late 1914, although the permissible strength of a 2nd-Line unit was only half of the normal establishment. This was raised to full establishment early in 1915, after which many of them were sent overseas, with some playing important parts in the fighting.

On 24 November 1914, it was decided to replace each overseas-service unit which proceeded abroad with its reserve unit (that is, the 2nd-Line now became available for home defence purposes); and directly this happened, a second reserve unit, or 3rd-Line, would be formed.

The 2nd-Line units were formed up into 2nd-Line Divisions, mirroring their senior counterparts. Most of them eventually served overseas, for the introduction of conscription in 1916 meant that all the men had been deemed to have agreed to serve overseas and the restriction that ckept them at home was removed: