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 in August and September 1914 into 1st Line (liable for overseas service) and 2nd Line (home service for those unable or unwilling to serve overseas) 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 (
2/1st Southern Mounted Brigade and
2/1st Western Mounted Brigade) without 1st Line antecedents were also formed.
2/1st Lowland Mounted Brigade was a mirror formation of the 1st Line
Lowland Mounted Brigade. and had under command the
2/1st Ayrshire (Earl of Carrick's Own) Yeomanry, the
2/1st Lanarkshire Yeomanry, and the
2/1st Lothians and Border Horse. By March 1916, the brigade was at
Dunbar,
East Lothian. On 31 March 1916, the remaining Mounted Brigades were ordered to be numbered in a single sequence and the brigade became
21st Mounted Brigade, still at Dunbar under
Scottish Command. ==Cyclist Brigade==