In November 1939 3rd S/L Rgt moved from Northern Ireland to Bordon Camp in Hampshire, and on 25 December it sailed from Southampton to join the British Expeditionary Force forming in France.

The battery on the coast at Dunkirk had two roles: AA defence of the town, and detection of aircraft attempting to drop Parachute mines in the approaches to the harbour.

[5][6][11] All of 3rd S/L Rgt's batteries were in action from the start of the campaign on 10 May, engaging low-flying attackers with LMGs by day, and illuminating bombers at night.

When the tanks of Army Group A cut through south of the BEF, 9 S/L Bty was rushed down to Arras to dig in and defend the perimeter with roadblocks.

[6] By now the regiment was scattered in sub-units from Vimy Ridge to Arras, but on 19 May it was ordered to move back to Dunkirk in independent groups.

The larger 150 cm S/L projectors had to be disabled and abandoned because there was no means of extricating them in the chaotic road conditions with the few vehicles available.

By 21 May, 9, 10 and 11 S/L Btys were in position for ground defence from Bergues to Gravelines along the outer line of canals covering the south-western approaches to the port, except for one party fighting a rearguard action with infantry near Blangy.

[6] On 23 May enemy troops bumped into positions held by 11 S/L Bty and were driven off by concentrated small arms fire.

Usher, 6th Battalion Green Howards with detachments of 3rd S/L Rgt defended the bridges at Gravelines and for three miles southwards.

[12][13][14] 11 Searchlight Bty was also attacked by tanks; they called down French artillery support, but much of the defensive fire fell short, making their positions untenable.

From Uxem it sent north to hold a canal entrance bridge and to man a small number of S/L projectors to illuminate the front against night attack – the first recorded use of this technique during the war.

While the rest of the regiment was being evacuated it was ordered to send a party to Veurne, about 10 miles (16 km) away on the Belgian border, and hold it against ground attack.

[17][18][19] The regiment supplied a cadre of experienced officers and men to 237th S/L Training Rgt at Holywood, County Down, where it provided the basis for a new 533 S/L Bty formed on 14 November 1940.

[29][30] 4th (Ulster) LAA Rgt left AA Command in April 1943 and became part of 21st Army Group, training for the planned Allied invasion of Normandy (Operation Overlord).

The HQ and advanced units of 101st AA Bde began landing in the American sector on 19/20 August to take over defence of Cherbourg Naval Base.

[32] At first, the AA units at Brussels were deployed to defend the liberated city against conventional piloted aircraft, but the air situation was quiet until 21 October.

[37][38][39][40] On 20 January 1958 76 Bty transferred to 24th Medium Rgt and the rest of the regiment had passed into suspended animation by the end of the month.