Primarily intended for home defence, it served in England and Ireland during Britain's major wars.

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.

[11][14] 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.

[22][23] The 5th Middlesex Militia (the Royal Elthorne Light Infantry) was formed the same month, giving Middlesex a total of five regiments:[23][24][25] War having broken out with Russia in 1854 and an expeditionary force sent to the Crimea, the militia began to be called out for home defence.

[34] The RSM returned home and was disembodied on 21 July 1856[19][20][21] A number of militia regiments were also called out to relieve regular troops required for India during the Indian Mutiny, and the Royal South Middlesex 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.

[23] 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).

[23] Lieutenant-Col Hon Charles Ernest Edgcumbe, formerly of the Grenadier Guards, was appointed Lt-Col Commandant of the battalion on 28 August 1880.

[14][19][20][21] The Royal Fusiliers established its regimental depot at Hounslow Barracks, which entailed no change for the 5th Bn, which had always been based in the town.

[23] During the late 1890s several regiments recruiting from large conurbations, including the Royal Fusiliers in London and its suburbs, were increased from two to four battalions.

[14][19][20][23][21] 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.

However, unlike a number of other militia battalions it did not serve in South Africa or in overseas garrisons, and was disembodied on 15 October the same year.

At the end of 1917 the 6th Bn went to Carrickfergus in Ireland, where it gave shelter to about 600 men of the American Expeditionary Forces rescued after their troopship, the SS Tuscania, was torpedoed of the Irish coast in early 1918.

Prior to 1881 the officers' helmet plate bore the Coat of arms of Middlesex (three Saxon Seaxes in pale).

[21][14][23][53][54] After retirement, the following Lieutenant-Colonels Commandant of the regiment were appointed as its Honorary Colonel:[23] The 6th (Reserve) Battalion is included in the inscription on the Royal Fusiliers War Memorial in High Holborn, London.