The British 21st Army Group was an important Allied force in the European Theatre of World War II.

Commanded by Field Marshal Sir Bernard Montgomery it initially controlled all ground forces in Operation Overlord. However, when sufficient American forces had landed, their own 12th Army Group was activated, under General Omar Bradley, and 21st Army Group was left with British Second Army and Canadian First Army.

After the succesful landings in the south of France by the US 6th Army Group, the 21st Army Group formed the left flank of the three Allied army groups arrayed against German forces in the west. It was therefore responsible for securing the ports upon which Allied supply depended. It also took part in one of the most famous operations in WWII.

After the break-out from Normandy, there were high hopes that the war could be ended in 1944. In order to do so, the last great natural defensive barrier of Germany in the west, the River Rhine, had to be breached. Operation Market Garden was the first attempt to do this. It was staged in Holland, with parachute divisions being dropped to capture bridges over the lower Rhine before they were blown by the Germans. The parachute formations would then be relieved by armoured forces pushing up northwards. However, the armoured formations would have only one road to use. Crucial information about the German formations in the area was also missing. The Heer divisions were much stronger than expected, thus giving the armoured units of British XXX Corps a much tougher fight than had been anticipated. The American parachute divisions that had taken part in the fighting were relieved in the end, but the British 1st Airborne Division was essentially destroyed.

Another high profile operation was mounted to clear the approaches to the great Belgian port of Antwerp. The island of Walcheren was strongly held by German forces, and commanded the estuary of the Meuse which flows through Antwerp. It was captured in late 1944 by the last major amphibious assault in Europe in WWII. A combination of Canadian forces and Royal Marines undertook the operation.

After the capture of Walcheren came the last great German offensive of the entire war. In a repeat of their 1940 attack, German formations smashed through Allied lines in the Ardennes in Belgium. As in 1940, the area was held weakly. However, unlike 1940, the Allies were able to blunt the attack and then destroy the forces that had mounted it.

The Battle of the Bulge presented a command problem to General Eisenhower. It had sliced through US lines, leaving some American formations both north and south of the new German salient. However, the headquarters of US 12th Army Group lay to the south, so US forces north of the salient were placed under 21st Army Group. They together with US Third Army under General George Patton reduced the salient.

After the battle, control of US First Army which had been placed under Montgomery's temporary command was returned to Bradley's 12th Army Group. However, US Ninth Army remained under Montgomery for the final attack into Germany.

British and American forces closed up to the Rhine completely by March 1945. Then, in one of the great lucky breaks of war, American forces captured an intact bridge across the Rhine at Remagen. British and Canadian forces crossed the river soon after, and German resistance in the west quickly crumbled. Canadian First Army wheeled left and liberated northern Holland, British Second Army occupied much of north-west Germany and liberated Denmark, and US Ninth Army helped to encircle the remaining German forces in the Ruhr pocket and on 4 April reverted to Bradley's 12th Army Group.

After the surrender, 21st Army Group was converted into the headquarters for the British zone of occupation in Germany. It was redesignated the British Army of the Rhine on 25 August 1945, and would eventually form the nucleus of the British forces stationed in Germany throughout the Cold War.