[2][3] The composition of the 3rd AA Division on the outbreak of war was as follows:[3][4] Mobilisation in the last week of August 1939 was difficult for the 3rd AA Division, which had the task of moving troops, guns and stores by road and by sea to remote and inaccessible sites in Orkney to defend the fleet anchorage at Scapa Flow, which had high priority.

[35] Unlike most of Britain's defence forces, the 3rd AA Division was frequently in action during the so-called Phoney War that lasted from September 1939 to May 1940.

The first action occurred unexpectedly on 16 October 1939, when nine enemy aircraft suddenly appeared out of cloud and dived on warships off Rosyth Dockyard, close to the Forth Bridge.

Among the vessels damaged in this raid was the Jutland veteran HMS Iron Duke, acting as a base ship and floating AA battery.

Starting in January 1940, the division was to receive 64 3.7-inch and 32 4.5-inch HAA guns and an increase to 100 searchlights, but only 10 Bofors and some Naval 2-pounders were available for LAA defence.

About 15 Junkers Ju 88s approached at low level in the dusk: half dived on the warships and the rest attacked the airfield.

On 4 April, a formation estimated at 12 Ju 88s carried out a series of medium- and low-level runs, dropping bombs and machine-gunning AA positions, and escaped without loss.

Also during 1940, all the searchlight units, whether AA battalions of the RE or still forming part of their parent infantry regiments, were transferred to the RA.

The increased sophistication of Operations Rooms and communications was reflected in the growth in support units, which attained the following organisation by May 1942:[64] The RAOC companies became part of the new Royal Electrical and Mechanical Engineers (REME) during 1942.