In accordance with the Territorial and Reserve Forces Act 1907 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 in August and September 1914 into 1st Line and 2nd Line units. Later, a 3rd Line was formed to act as a reserve, providing trained replacements for the 1st and 2nd Line regiments. Similarly, by 1915 most 2nd Line yeomanry regiments were formed into 2nd Line mounted brigades with the same title and composition as the pre-war 1st Line formations. Two other 2nd Line brigades without 1st Line antecedents were also formed. 2/1st Welsh Border Mounted Brigade was a mirror formation of the 1st Line Welsh Border Mounted Brigade. It was formed in the Newcastle area of Northumberland in January 1915 under the command of Brigadier-General M.O. Little. It had under command the 2/1st Shropshire Yeomanry, the 2/1st Cheshire Yeomanry, and the 2/1st Denbighshire Hussars and was placed under the command of the 63rd Division. On 31 March 1916, the remaining Mounted Brigades were ordered to be numbered in a single sequence and the brigade became 17th Mounted Brigade, still in Northumberland under Northern Command. In April 1916, the brigade joined the 1st Mounted Division in East Anglia where it replaced its 1st Line which had departed for Egypt. At this time, the brigade also commanded 2/1st Shropshire Royal Horse Artillery and ammunition column, a signal troop, a Field Ambulance, RAMC, a mobile veterinary section and a Transport and Supply Column, ASC. By July it had left for the Morpeth, Northumberland area.
Cyclist Brigade
In July 1916 there was a major reorganization of 2nd Line yeomanry units in the United Kingdom. All but 12 regiments were converted to cyclists and as a consequence the brigade was converted to 10th Cyclist Brigade. Further reorganization in October and November 1916 saw the brigade redesignated as 6th Cyclist Brigade in November, still in the Morpeth area. At this time the 2/1st Denbighshire Yeomanry departed for the 1st Cyclist Brigade at Beccles in Suffolk where it was amalgamated with the 2/1st Montgomeryshire Yeomanry as the 3rd Cyclist Battalion. It was replaced by the 2/1st Fife and Forfar Yeomanry, which was stationed at Ashington. In March 1917, 2/1st Shropshire Yeomanry moved to Newbiggin, and later to Woodhorn near Morpeth. In July, 2/1st Fife and Forfar Yeomanry and 2/1st Cheshire Yeomanry moved to Acklington. Early in 1918 the Brigade moved to Ireland and was stationed at the Curragh. It was joined there by the 2/1st Queen's Own Dorset Yeomanry and the 2/1st Essex Yeomanry. There were no further changes before the end of the war.
