Between 1905 and 1912 the Territorial Force (after 1921, Territorial Army) and Special Reserve were established. The army was greatly increased in size by conscription during World War I.

The Kitchener Armies

At the start of WWI, Field Marshal Lord Kitchener was appointed Secretary of State for War of 6th August and the next day he issued his call for the first hundred thousand. These men would form the first divisions of the 70 divisions Kitchener planned to raise to fight the war. This would be a new army, composed of volunteers. Each man would sign up for three years or the duration, whichever was shorter, and would agree to being sent to serve anywhere.

Within fourteen days the first hundred thousand were recruited, and on 21st August an Army Order was issued to form six divisions, one from each of the geograghical Commands the country was divided.Thus, the First New Army (K.1) was formed comprising 9th to 14th Divisions, each with a (geograghical) title in addition to their number.

A Second New Army (K.2) was authorised on 11th September with divisions numbered 15 to 20 with the same geograghical designation as K.1.

The Third New Army was formed only three days later. K.3's divisions were numbered 21 to 26, without any designation.

A Fourth New Army was formed in October from Reserve and Special Reserve battalions of the Line Regiments. The divisional numbers were 30 to 35, since 27, 28, 29 were already in use for divisions formed from regular battalions. Later, however a decision was taken to re-convert the battalions of K.4 into Reserve units, breaking up the Divisions, and creating Training Reserve Brigades.

Eventually enough men had come forward that on 10 December 1914, the order was issued to create a Fifth New Army. It's six Divisions were initially numbered 37 to 42. This was renumbered K4 when the original K4 was broken up, and the Divisions took the original numbers 30 to 35. Most of the units of this Army were locally raised, often referred to as pals.

The Fifth New Army (initially raised as the sixth and later renumbered) was formed in December 1914 with the next six numbers, 37 to 42. This army comprised locally raised battalions, formed by committees from cities, towns, organizations and individuals.These included the Ulster and Welsh Divisions, and the Pals battalions, composed of men with the same geographical, social or occupational background.  One such regiment was the 23th (Service) Battalion The Duke of Cambridge's Own (2nd Football) into which young Arthur William Harding have volunteered. He died of wounds four days after Armistice on 15 Nov 1918 aged 19.

Thus, within one year thirty New Army divisions and six training divisions had been raised, all as result of Kitchener's call to arms that started 6th August 1914. In all 557 battalions were formed.