Introduction
In 1922, with the partition of Ireland, Northern Ireland District was formed with its headquarters based in Belfast. The role of Northern Ireland District was internal security, and the defence of the Province. It was a Major General’s command.

There were three Regular Army regiments with their depots located in the Province:

Omagh
The Royal Inniskilling Fusiliers;
The Royal Irish Fusiliers (Princess Victoria’s)

Armagh
The Royal Ulster Rifles

The outbreak of the Second World War – September 1939
With the outbreak of the Second World War, the establishment of the District was increased. This was achieved in the main by the use of retired officers from the Regular Army Reserve of Officers. The four Regular Army battalions stationed in Northern Ireland were all quickly posted away, to be replaced by the Territorial Army 53 (Welsh) infantry Division, which arrived in tranches between October 1939 and April 1940. In November 1940, the Northern Ireland District was divided into three areas:

Belfast;
Northern Area;
Southern Area.

The new officer commanding the Belfast Area was appointed in November 1940, but the other two areas did not have commanders appointed until March and April 1941. By April 1940, the whole of the 53 Infantry Division was deployed to the Province. It was followed by the 61 Infantry Division which arrived on 20 June 1940. With two field formations now deployed in the Province, on 12 July 1940, a new appointment was made as General Officer Commanding British Troops in Northern Ireland (B.T.N.I.), a Lieutenant General’s position. The Headquarters, VI Corps was formed in the Province, however, the corps appears not to have functioned as an operational formation being absorbed by B.T.N.I.. The role of B.T.N.I. was to prepare the Province in case of the any threat of invasion from the Republic of Ireland. Northern Ireland District remained in being to control non-field force units and depots in the Province.

British Troops in Northern Ireland – 1941 – 1945

The 5 Infantry Division arrived in the Province in April 1941. Headquarters III Corps was sent to the Province in June 1941, to take command of the 5 and 53 Infantry Divisions. The 53 Infantry Division returned to the mainland in November 1941, and the 5 Infantry Division left in early 1942 to be deployed overseas. The 61 Infantry Division left Northern Ireland on 2 February 1943 to return to XI Corps District on the Mainland. Headquarters III Corps left the Province on 31 May 1942, to commence its move to Persia and Iraq. The 59 (Staffordshire) Infantry Division transferred from III Corps to B.T.N.I. on this date. This division remained in Northern Ireland until 19 March 1943 when it returned to the mainland.

The first U.S. Troops arrived at Belfast in January 1942. On 7 May 1942, Major General Russel P. HARTLE, assumes command of the U.S. Army Northern Ireland Force. In November 1942, most U.S. troops left Northern Ireland on being deployed on Operation Torch, the invasion of French North Africa. In autumn 1943, U.S. troop levels began to build again in Northern Ireland in anticipation of the invasion of France. Most American forces left in June 1944, with the last U.S. Army unit leaving on 7 July 1944 for France.

On 9 May 1943, the Headquarters B.T.N.I. was amalgamated with the Northern Ireland District to form one combined headquarters, based at Lisburn. This was because the threat from the Republic of Ireland has disappeared.  On 21 December 1943, the H.Q. of the 55 Infantry Division arrived in the Province to come under command of B.T.N.I.. The divisional headquarters and two brigades returned to the mainland in July 1944, but the 199 Infantry Brigade remained under command of Northern Ireland District until June 1945, when it left to rejoin the rest of the division. In 1944, the areas disbanded, and B.T.N.I. was downgraded back to a District establishment.