The British Second Army was extant in both World Wars.

World War I

During World War I, the army was formed on December 26, 1914 when the British Expeditionary Force was split in two due to becoming too big to control its subordinate formations. The army was originally commanded by General Horace Smith-Dorrien and later by General Herbert Plumer. It spent the most of the war around the Ypres salient but was moved to Italy between November 1917 and March 1918.

World War II

The World War II formation was commanded by Lieutenant General Sir Miles Dempsey and served under 21st Army Group. Two of its formations, I Corps and XXX Corps took part in the D Day landings of Operation Overlord. The initial penetration on D Day was not as good as hoped, and this pattern was repeated during the rest of the Normandy campaign. A third British corps, VIII Corps, entered the line during June to add its weight to the assault. However, the main British objective of this part of the campaign, Caen, still did not fall. Second Army mounted several offensives to attempt its capture. The city finally fell at the end of June.

By the end of July, American forces had broken out of Normandy. As the swept east, the German Seventh Army was pinned by Second Army and trapped in pocket around Falaise. The formation was subsequently annihilated. Second Army then commenced a dash across France in tandem with the Americans on its right, and the Canadians on its left. During the interim, I Corps had left its control and been assigned to the Canadian First Army, and the British XII Corps had come into the line to replace I Corps in Second Army.

Second Army entered Belgium quickly, and cleared much of the country. Its captures included the capital Brussels and the great port of Antwerp

Second Army's most high profile operation in 1944, apart from Overlord was Operation Market Garden. British and American parachute troops were landed to capture vital bridges over several rivers in order to allow the Allied troops to cross the Rhine and advance into Germany. Second Army's XXX Corps was then supposed to push up a single road and relieve the parachute troops. However, the single road caused enormous logistical difficulties, and crack German troops that intelligence was unaware of were in the area. The American formations were relieved, but the British 1st Airborne Division, after holding the bridges at Arnhem far longer than had been specified in the plan, and even beyond the best estimates of what was possible, was largely destroyed before it could be relieved.

After the failure of the operation, Second Army spent the rest of the year exploiting the salient that it had created to close up to the Rhine and Meuse rivers in Holland. The final part of that phase too place in mid-January 1945, with the clearing of the Roermond Triangle by XII and VIII Corps. This enabled the completion of closing up to the River Roer .

During February, Second Army entered a holding phase. Whilst it pinned down the German forces facing it, the Canadian First Army and US Ninth Army made a pincer movement from north and south which piered the Siegfried Line in that area and cleared the remaining German forces west of the Rhine in conjunction with further American offensives in the south of the Rhineland.

Second Army crossed the Rhine on 23 March. It then headed across the North German Plain, with First Army on its left wheeling to clear the north of Holland, and Ninth Army on its right helping to trap the German Army Group B under Model in an enormous pocket. With Army Group B trapped, the last major German formation in the west was out of the equation, and the German state began its final disintegration.

Second Army reached the Weser on 4 April, the Elbe on 19 April on by 7 May the Soviet Army had met up with the British forces and the shore of the Baltic Sea had been reached. Shortly thereafer, the Second World War came to an end with the surrender of the government of Karl Dönitz.