At the outbreak of the
First World War, the
Northumberland Fusiliers consisted of seven battalions:
The expansion in battalions mostly came through two sources: the duplication of the Territorial Force battalions and the formation of
New Armies. Of the 45 battalions raised during the war, 10 were Territorial Force and 27 were New Army.
In accordance with the
Territorial and Reserve Forces Act 1907 (
7 Edw. 7, c.9) which brought the
Territorial Force into being, the TF was intended to be a home defence force for service during wartime and members could not be compelled to serve outside the country. However, on the outbreak of war on 4 August 1914, many members volunteered for
Imperial Service. Therefore, TF units were split into 1st Line (liable for overseas service) and 2nd Line (
home service for those unable or unwilling to serve overseas) units. 2nd Line units performed the home defence role, although in fact most of these were also posted abroad in due course. Later, a 3rd Line was formed to act as a reserve, providing trained replacements for the 1st and 2nd Line units.
 When 2nd Line battalions were formed, the 1st Line took on a fractional designation so, for example,
4th Battalion became
1/4th Battalion (first fourth) and its 2nd Line was designated
2/4th Battalion (second fourth); in due course the 3rd Line was formed as the
3/4th Battalion (third fourth).
In the summer of 1915, personnel of 2nd and 3rd line battalions who had not volunteered for overseas service were formed into Provisional Battalions. They were used to form Provisional Brigades and later Home Service divisions (
73rd Divisions); on 1 January 1917 they became numbered battalions of line infantry regiments.
 In the case of the Northumberland Fusilers, the 21st and 22nd Provisional Battalions became the
36th Battalions (T.F.) of the regiment.
Kitchener was one of the few people in 1914 to realize that the war was not going to be a short one; he believed that it would last three years and would require an army of 70 divisions. He eschewed the Territorial Force – partly due to the limitations imposed by its terms of service but also due to the poor impression he formed when observing the French Territorials in the
Franco-Prussian War – and did not make use of the framework envisioned by Haldane. He launched his appeal for 100,000 volunteers on 7 August 1914 to form a New Army of six divisions (and support units) and within a few days this target had been reached; by the end of September, half a million volunteers had come forward to form the New Armies.
Each of the 69 line infantry regiments raised one battalion for the
[d] and for the
Second New Armies (K2)
[e] designated as Service battalions and numbered after the existing Territorial Force battalions (so
9th (Service) Battalions for the Northumberland Fusiliers). This rigid structure did not take account of the differing ability of regiments to raise troops based upon the population of their recruiting areas. Therefore, the
Third New Army (K3) had a much higher proportion of battalions from the more populous north of England, notably Cheshire, Lancashire, Yorkshire, Durham and Northumberland (
14th (Service) Battalions). The
Fourth New Army (K4) was formed from men of the Reserve and Special Reserve battalions which were over establishment. Originally formed into the 30th – 35th Divisions, these were broken up so the battalions could train recruits and send drafts to the first three New Armies.
 The regiment raised the
15th (Reserve) Battalion in this manner.
While the first four New Armies were being raised, a number of units were also being raised by committees in cities and towns, and by other organizations and individuals – the
Pals battalions. These were housed, clothed and fed by their committees until the
War Office took them over in 1915 and the raisers' expenses were refunded. These units formed the
Sixth New Armies (later called the new Fourth and Fifth New Armies when the original Fourth New Army was broken up).
 The Northumberland Fusiliers raised the largest number of pals battalions of any regiment,
[f] notably including the
Tyneside Scottish and
Tyneside Irish Brigades:
17th (NER Pioneers),
18th (1st Tyneside Pioneers),
19th (2nd Tyneside Pioneers),
20th (1st Tyneside Scottish),
21st (2nd Tyneside Scottish),
22nd (3rd Tyneside Scottish),
23rd (4th Tyneside Scottish),
24th (1st Tyneside Irish),
25th (2nd Tyneside Irish),
26th (3rd Tyneside Irish), and
27th (4th Tyneside Irish) battalions.
The locally recruited – pals – battalions formed depot
companies and in 1915 these were grouped into
local reserve battalions to provide reinforcements for their parents. The regiment formed
29th (Tyneside Scottish),
30th (Tyneside Irish),
33rd (Tyneside Scottish), and
34th (Tyneside Irish) Reserve Battalions.
The regiment formed eight other battalions. The
37th (Home Service) Battalion was formed in April 1918 to replace the
36th Battalion when it moved to the
Western Front. The
38th Battalion was formed in June 1918, but was absorbed into the
22nd Battalion before the end of the month.
The regiment raised three Garrison Battalions (
3rd) of officers and men unfit for active service but considered fit for garrison duty at home and overseas, thereby releasing fitter troops for front line service.
In May 1917, the
Training Reserve was reorganized with the battalions becoming more specialized in their training. Young Soldier Battalions took in recruits aged 18 years and one month and, after basic training, posted them to one of two linked Graduated Battalions. Eventually, 23 Young Soldier Battalions and 46 Graduated Battalions were formed. On 27 October 1917, these were allocated to 23 infantry regiments and thereby the Northumberland Fusiliers gained the
52nd (Graduated) and
53rd (Young Soldier) Battalions.
A number of reserve battalions served during the war. They recruited and trained drafts for the active service units and were designated by use of (Reserve) after the battalion number. These had a number of different origins and had a variety of fates.
Special Reserve battalions
Childers Reforms of 1881 created regimental districts, each allocated a two-battalion regiment, usually bearing a "county" title. Existing two-battalion
(1st to 25th inclusive) were redesignated, whereas the single-battalion foot regiments were paired to become the 1st or 2nd battalions of the new regiments. At the same time the existing
rifle volunteer units of the district became battalions of their regiments, the militia numbered after the regulars – thus 3rd (Millitia) Battalion, 4th (Millitia) Battalion, etc. – and the volunteers in a separate sequence – 1st Volunteer Battalion, 2nd Volunteer Battalion, etc. As the
5th (Northumberland) (Fusiliers) Regiment of Foot already had two battalions, it simply became the
 as the county regiment of
Northumberland. The Northumberland Light Infantry Militia became the 3rd (Militia) Battalion and the rifle volunteers formed the 1st, 2nd and 3rd Volunteer Battalions of the regiment.
reforms by Haldane in 1908 saw the militia transferred to a new "
Special Reserve" as "Reserve" – 3rd battalions – or "Extra Reserve" – 4th and subsequent – battalions. The volunteer battalions were renumbered in the same sequence as the regulars and millitia. Hence, the 3rd (Millitia) Battalion became the
3rd (Reserve) Battalion and the volunteers became the 4th, 5th and 6th Battalions of the Northumberland Fusiliers.
Almost all of the Special Reserve battalions remained in the United Kingdom throughout the war,
[g] training replacements and providing drafts to the regular battalions.
 The 3rd (Reserve) Battalion remained part of the regiment as a training unit until the end of the war.
2nd Reserve battalions
On 8 October 1914, each Reserve and Extra Reserve battalion of the line infantry regiments were instructed to form a Service battalion. The battalions thus raised, including the
15th (Service) Battalion, were used to form the divisions of Kitchener's
Fourth New Army.
 The divisions were never fully formed; the need for trained reinforcements for the first three New Armies meant that they were broken up and the infantry battalions were used to provide and train reinforcements. On 10 April 1915, the infantry battalions became reserve formations to be known as 2nd Reserve battalions (in the sense that they were second to the original Reserve and Extra Reserve battalions).
 The 15th Battalion became the 15th (Reserve) Battalion and provided replacements for the 8th – 14th battalions.
The introduction of conscription caused a reorganisation of the reserve battalions as the regimental system could not cope with the number of new recruits. A new, centralized, system was put in place, called
The Training Reserve. On 1 September 1916, the 15th Battalion became part of the new organisation.
Local Reserve battalions
The locally recruited Service battalions of the
Sixth New Armies – the
Pals battalions – formed depot
companies and in 1915 these were grouped into Local Reserve battalions to provide reinforcements for their parents. Like the 2nd Reserve battalions, they became part of the Training Reserve on 1 September 1916.
Territorial Force Reserve battalions
Almost all Territorial Force battalions had formed a 3rd Line – designated with the fractional 3/ – by June 1915; they supplied reinforcements to their parent 1st and 2nd Lines. In the autumn of 1915 they were brought together in 14 Third Line Groups, one for each of the pre-war T.F. divisions
 – those of the Northumberland Fusiliers joining the Northumbrian Third Line Group,
 along with those of the
East Yorkshire Regiment,
 and the
Durham Light Infantry.
In April 1916 they dropped the fractional designation and became Reserve Battalions T.F. On 1 September 1916 the reserve territorial battalions of each regiment were amalgamated into a single unit (or two units in certain large regiments, for example the
Manchester Regiment's 5th and 8th Reserve Battalions
). At the same time, the Third Line Groups were renamed as Reserve Brigades T.F.
 The Northumbrian Reserve Brigade T.F. commanded 4th (Reserve) Battalion, Northumberland Fusiliers,
 4th (Reserve) Battalion, East Yorkshire Regiment,
 4th (Reserve) Battalion, Green Howards,
 and 5th (Reserve) Battalion, Durham Light Infantry
 in the
Hornsea area as part of the Humber Garrison.
Along with the
Special Reserve battalions, the T.F. Reserve battalions remained as regimental reserves. New recruits were posted to them first until they were up to strength, then to the Training Reserve. When drafts were needed for the overseas units, they were taken first from the regimental reserves; if there were insufficient trained replacements then recourse was made to the Training Reserve.