The battalion's origins lay in the enthusiasm for joining local Rifle Volunteer Corps (RVCs) engendered by an invasion scare in 1859.

[8][9] The unit was renumbered the 5th (West Middlesex) Middlesex RVC on 3 September 1880 and was attached as a Volunteer Battalion (VB) to the Royal Fusiliers on 1 July the following year, transferring to the King's Royal Rifle Corps (KRRC) as the 4th VB in July 1883, without changing its title in either case.

[4][2][7][9][10] The Stanhope Memorandum of December 1888 proposed a Mobilisation Scheme for units of the Volunteer Force, which would assemble by brigades at key points in case of war.

[14][15][16][17] During the period of tension before the outbreak of war, the 9th Battalion sent two special service sections to guard a cable station at Cuckmere Haven and Birling Gap (28 July).

When the mobilisation orders were received on 4 August 1914, the Home Counties Division was on the march from Aldershot to Salisbury Plain for its annual training.

[14][18] On 11 August, in common with the majority of the men of the Home Counties Division, the 9th Battalion accepted liability for overseas service.

[21][22] In May and August 1915, the battalion supplied its first drafts to the 2nd Bn Norfolk Regiment serving in Indian Expeditionary Force D in Mesopotamia; of the 50 other ranks sent, 20 died at the Siege of Kut or in captivity afterwards.

During this period, it was constantly moving station:[23][24] From November 1917, the battalion reverted to the title of 9th Middlesex when the 2/9th Bn disbanded in England (see below).

[14][15][16][26][27][28] The division was not concentrated until mid-March 1918, and when it moved north up the Tigris, 53rd Brigade was left behind to subdue Nejef, south of Baghdad.

After making a forced march of nearly 90 miles in a week through almost waterless country and carried out a demonstration, the brigade was being withdrawn when trouble flared up again.

Summer weather made campaigning impractical in Mesopotamia, so the division was engaged in roadbuilding until the beginning of October, when orders were received to join the renewed advance up the Tigris.

The brigade passed through the Fathah Gorge and the following day pushed on under artillery and machine-gun fire to establish a bridgehead over the Little Zab.

18th Indian Division pushed a flying column on to capture Mosul, while the infantry retired towards its railhead at Baiji for supplies.

After the Armistice of Mudros came into effect on 31 October, the division began preparing for the postwar occupation of Iraq as the Turks withdrew.

[14][37][38] Demobilisation began in early 1919, but on 23 May the battalion (now reduced to three companies) was ordered to join a punitive column marching into Kurdistan.

The 67th Division had dual responsibility as part of the mobile force for Home Defence, and to train drafts for overseas service.

[4][7][42] For three years from January 1927 the battalion's Regular Army adjutant was Capt Brian Horrocks while he was studying for entry to the Staff College, Camberley.

[4][7][44][45][46][51][52] By that time – the height of the Battle of Britain – the regiment had been transferred within 2 AA Division to 41st (London) Anti-Aircraft Brigade, which had special responsibility for defending Royal Air Force airfields in East Anglia.

[53][54][55][56] The regiment supplied a cadre of experienced officers and men to 231st S/L Training Rgt at Blandford Camp where it provided the basis for a new 523 S/L Bty formed on 14 November 1940.

In January 1944, it moved to Bournemouth to take over Air Defence of Great Britain commitments around Boscombe, Poole and Swanage.

It handed over these commitments in late February and moved to No 11 LAA Training Camp at Stiffkey, followed by deployment exercises in Lincolnshire.

[58][61] After the bulk of the invasion force had embarked, 126 Rgt's batteries were trained at Larkhill Camp in engaging unseen ground targets.

Ordered to its embarkation marshalling area in August, the regiment was diverted to Pevensey, Hove and Bexhill-on-Sea to engage incoming V-1 flying bombs as part of Operation Diver.

[66] From 15 November 1944, the regiment defended the Nijmegen bridges against air and waterborne attack under command of 74 AA Bde and then of 1st Canadian Infantry Division.

On 17 December, after weeks of quiescence, the Luftwaffe made a major effort to support its surprise attack in the Ardennes (the Battle of the Bulge).

In 21st Army Group's sector, the Maas and Waal bridges were attacked by waves of Bf 109 and Fw 190 fighter-bombers operating at low level (below radar and HAA), which had to be engaged by LAA guns.

Improved radar techniques were allowing effective LAA barrages to be fired, driving the attackers back to higher levels into the range of HAA guns.

When the TA was converted into the smaller TAVR in 1967, 873 Sqn was reduced to HQ and one Troop – the only dedicated searchlight unit remaining not only in the British Army but the whole of NATO.

A regimental arm flash was worn during World War II, consisting of a diamond divided vertically in maroon and gold.

[46][51] The unit's first stand of Regimental colours, presented by HRH Princess Beatrice of Battenburg in 1910 and laid up in 1950, and their replacements presented that year by Lt-Gen Sir Brian Horrocks (the battalion's prewar adjutant), are preserved in the Middlesex Regiment Chapel in St Paul's Cathedral.