[1][2][3][4] At the time the brigade was formed, the TA's AA units were in a state of mobilisation because of the Munich crisis, although they were soon stood down.

In mid-May, as the Battle of France got under way, 45 AA Bde's units – particularly the widely spaced S/L sites – were ordered to find rifle detachments to guard against possible attacks by German paratroopers.

[8] After the British Expeditionary Force was evacuated from Dunkirk, the German Luftwaffe began almost nightly minor air raids, often by single aircraft, against the dock facilities and steelworks of South Wales, though the Battle of Britain was mainly fought over Southern England.

[8] In July 1940, the Cardiff area was protected by a mere 12 HAA guns,[17] but during the summer the AA defences of South Wales were bolstered by a number of units that had been re-equipped after evacuation from Dunkirk and Norway.

The arrival in September of the Regular Army 1st S/L Rgt, RA, also re-equipped after Dunkirk, allowed 45 AA Bde to complete the illuminated areas of South Wales, and 484 (Carmarthenshire) S/L Bty was relieved and sent to Devonport.

The available LAA units were distributed to defend Vulnerable Points (VPs) such as docks and Glascoed Royal Ordnance Factory.

At this time, 45 AA Bde was deployed as follows:[8] There were frequent moves of sub-units as reinforcements were integrated into the defences, and as opportunities arose for training.

At first the towns of South Wales, including important coal and oil port facilities, refineries, steelworks and ordnance factories, were under almost nightly air attack, to which the AA defences replied as best they could.

In the absence of effective GL radar control, at night the guns could only reply blindly with fixed barrages.

[9][27][54][55] The HAA and support units increasingly became 'Mixed', indicating that women of the Auxiliary Territorial Service (ATS) were fully integrated into them.

At the same time, experienced units were posted away to train for service overseas (sometimes being lent back to AA Command while awaiting embarkation).

[27][60][61] During this period the brigade was composed as follows:[42][62][63][64][65] In March 1944 AA Command undertook a major reorganisation in order to defend the assembly camps, depots and embarkation ports for the planned Invasion of Normandy (Operation Overlord) and to prepare for the expected onslaught of V-1 flying bombs (codenamed 'Divers') against London.

At the same time the Luftwaffe was suffering from such shortages of pilots, aircraft and fuel that serious air attacks on the rest of the UK could be discounted.