The 100th Light Anti-Aircraft Regiment, Royal Artillery, (100th LAA Rgt) was an air defence unit of the British Army during World War II.

It served with 56th (London) Infantry Division in the final stages of the Tunisian Campaign, in the landings at Salerno and subsequent fighting in Italy, including the Battle of Anzio, until it was disbanded at the beginning of 1945.

The unit was originally formed in Devonshire on 21 March 1940 as 18th (Pioneer) Battalion, Royal Fusiliers, as part of the rapid expansion of the Army with wartime conscripts.

This was a prewar Territorial Army (TA) motorised division serving in XII Corps in invasion-threatened East Kent, with 18th Royal Fusiliers at Lydd and Old Romney.

From South Africa most of the troops sailed to Bombay, then on to Basra in Iraq, arriving on 4 November and then by road and rail to Kirkuk.

Then it began an overland drive to join Eighth Army in Tunisia, travelling some 3,200 miles (5,100 km) between 18 March and 19 April.

In the rough country of Tunisia the forward LAA units were often involved in 'snap' engagements against fast, low-flying air attacks.

There was bitter fighting for 10 days in the beachhead and the AA units frequently found themselves in the firing line holding off German attacks that threatened to break through 56th (L) Division's front.

[19][20][21] The Luftwaffe was active in trying to prevent the Allies from crossing the Volturno, particularly with Messerschmitt Bf 109 and Focke-Wulf Fw 190 single-seat fighter-bombers, and 12th AA Bde came up to reinforce the divisional LAA units along the river.

56th (L) Division was about to resume the offensive on the Garigliano on 30 January when it was hurriedly withdrawn to reinforce the landing further up the coast at Anzio, which had run into trouble.

Its 90th LAA Rgt landed a SP Bofors battery with the first wave and the whole regiment was ashore by D + 5 to defend the field artillery positions.

The Germans quickly contained the beachhead and by 1 February were driving the Allied troops back towards the sea, and sending over waves of air attacks.

The CO of 90th LAA Rgt asked for help and got some mobile No 4 Mark III lightweight local warning radar sets from 22nd AA Bde back at Salerno.

On 10 July the refitted division left Port Said to return to Taranto, from where it was sent to join V Corps with Eighth Army on the Adriatic coast of Italy.

As early as June 1944 the Chiefs of Staff had decided that the number of AA regiments in Italy must be reduced and their fit personnel converted to other roles, particularly infantry.