It was converted to the Special Reserve under the Haldane Reforms and supplied reinforcements to the Royal Fusiliers' fighting battalions during World War I.

The universal obligation to military service in the Shire levy was long established in England and its legal basis was updated by two acts of 1557 (4 & 5 Ph.

It was an important element in the country's defence at the time of the Spanish Armada in the 1580s, and control of the militia was one of the areas of dispute between King Charles I and Parliament that led to the English Civil War.

[1][2][3] Under threat of French invasion during the Seven Years' War a series of Militia Acts from 1757 reorganised the county militia regiments, the men being conscripted by means of parish ballots (paid substitutes were permitted) to serve for three years.

[10][11][12] During the French wars, the militia were embodied for a whole generation, and became regiments of full-time professional soldiers (though restricted to service in the British Isles), which the Regular Army increasingly saw as a prime source of recruits.

The recruiting area of the 2nd Royal West Middlesex Militia was effectively split, with the new 5th Regiment taking over the north-western part of the county in Elthorne Hundred (one of the ancient subdivisions of the County of Middlesex, centred on Uxbridge and roughly coinciding with the modern London Borough of Hillingdon) while the existing 2nd regiment took over Edmonton Hundred, the most northerly division of Middlesex, then centred on Barnet.

[24][28] By November the regiment had joined the Royal East Middlesex Militia at Aldershot Camp, where the huts were only just completed.

During the early part of 1856 the militia regiments concentrated at Aldershot carried out collective training, and were reviewed by Queen Victoria.

The war ended when the Treaty of Paris was signed on 30 March: the concentration at Aldershot was broken up in June and regiments returned to their headquarters.

[22][21][29][30] A number of militia regiments were also called out to relieve regular troops required for India during the Indian Mutiny, and the Royal Elthorne LI was embodied on 1 October 1857.

The Militia Reserve introduced in 1867 consisted of present and former militiamen who undertook to serve overseas in case of war.

[24][39] Under the 'Localisation of the Forces' scheme introduced by the Cardwell Reforms of 1872, the militia were brigaded with their local Regular and Volunteer battalions.

The militia now came under the War Office rather than their county lords lieutenant and battalions had a large cadre of permanent staff (about 30).

The 5th Middlesex Militia's assigned war station was with the Garrison Army in the Sheerness Division of the Thames and Medway defences.

Instead the regulars used Woolwich while the militia used Warley Barracks in Essex, the 5th Middlesex retaining its regimental HQ at Uxbridge.

[22][21][20][24] After the disasters of Black Week at the start of the Second Boer War in December 1899, most of the regular army was sent to South Africa, the militia reserve was called out to reinforce them, and many militia units were embodied to replace them for home defence and to garrison certain overseas stations.

The 5th Middlesex was embodied again on 6 January 1902, and volunteered for overseas service[22][21] The battalion embarked for South Africa with a strength of 27 officers and 825 other ranks under the command of Lt-Col Vilett Rolleston.

It arrived at Cape Town on 15 March and went to Vryburg, and then to Mafeking with detachments at Maribogo, Kraaipan and Maritzani.

[22][21] During its short service in South Africa the battalion lost eight non-commissioned officers and men killed in action or died of disease.

[45][46] Under the sweeping Haldane Reforms of 1908, the militia was replaced by the Special Reserve, a semi-professional force similar to the previous militia reserve, whose role was to provide reinforcement drafts for regular units serving overseas in wartime.

Once the pool of reservists had dried up, the 5th Bn trained thousands of raw recruits for the active service battalions.

The officers' and men's buttons had the Roman numeral V between the strings of a bugle-horn, contained within a crowned garter inscribed with the title, the whole superimposed on a silver cut star.