21st Army Group The 21st Army Group was a British headquarters formation , in command of two field armies and other supporting units , consisting primarily of British and Canadian forces . Established in London during July 1943 , under the command of Supreme Headquarters Allied Expeditionary Force ( SHAEF ) , it was assigned to Operation Overlord , the Western Allied invasion of Europe , and was an important Allied force in the European Theatre of the Second World War . The 21st Army Group operated in Northern France , Luxembourg , Belgium , the Netherlands and Germany from June 1944 until the end of the war in Europe in 1945 , after which it was redesignated the British Army of the Rhine ( BAOR ) . Normandy . Commanded by General ( later Field Marshal ) Sir Bernard Montgomery , 21st Army Group initially controlled all ground forces in Operation Overlord ( the United States First Army and British Second Army ) . When sufficient American forces had landed , their own 12th Army Group was activated , under General Omar Bradley , and the 21st Army Group was left with the British Second Army and the newly activated First Canadian Army . Normandy was a battle of attrition for the British and Canadian troops , drawing in most of the available German reinforcements , espcially armoured divisions , around Caen at the eastern end of the lodgement . The Germans lacked the ability to prevent the American breakout at the western end of the Normandy beachhead in early August 1944 . Following the German attack towards Mortain , the American breakout and an advance by the 21st Army Group the German armed forces in Normandy were nearly enveloped in the Falaise pocket , and subsequently routed retreating towards the Low Countries . Advance into the Netherlands . After the successful landings in the south of France by the U . S . 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 , and also with overrunning German V - 1 and V - 2 launching sites along the coasts of western France and Belgium . By 29 August , the Germans had largely withdrawn across the Seine River without their heavy equipment . The campaign through Northern France and Belgium was largely a pursuit , with the ports - formally designated " Fortress Towns " by the Germans - offering only limited opposition to the First Canadian Army . The advance was so rapid , 250 miles in four days , that Antwerp , Belgium was captured undefended on 4 September 1944 and the port facilities were cleared of the German defenders in the following days . On 1 September 1944 , the 21st Army Group was relieved of operational control of the American armies , and those armies formed the 12th Army Group . By mid - September , elements of 21st Army Group had reached the Dutch border , but were halted due to lack of supplies , and by flooding caused by the widespread German demolition of Dutch dikes . German control of some of the channel ports and the approaches to Antwerp , and previous Allied bombing of the French and Belgian railways , resulted in a long supply line from Normandy served mainly by trucks . Operation Market Garden . 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 Rhine River had to be crossed . Operation Market Garden was orchestrated to attempt just this . It was staged in the Netherlands with two American and one British airborne divisions and a Polish parachute brigade being dropped to capture bridges over the lower Rhine before they were blown by the Germans . The airborne formations were then to be relieved by armored forces advancing rapidly northwards through Eindhoven and Nijmegen to Arnhem , opening the north German plains , and the industrial Ruhr Valley , to the Allies . However , the British armored forces had only one main highway to operate on , and crucial information about the German forces in the operational area was either missing or ignored . The scratch forces remaining after the retreat from France were much stronger than expected , thus giving the armored units of the XXX Corps a much tougher fight than had been anticipated , slowing the advance . The American divisions and the Polish parachute brigade that had fought south of the Rhine were relieved but the British 1st Airborne Division in Arnhem was practically destroyed . Battle of the Scheldt . Since the approaches to the port of Antwerp had not been cleared when the city was captured it had allowed the German army time to reorganise and dig in along the approaches making the port completely unusable . Thus an operation was needed to clear the approaches and thereby ease the supply problem . The island of Walcheren was strongly held by German forces and commanded the estuary of the Scheldt which flows through Antwerp . Operations by II Canadian Corps cleared the approaches to Antwerp both north and south of the water during the Battle of the Scheldt . Walcheren itself was captured in late 1944 by the last major amphibious assault in Europe in the Second World War . A combination of Canadian forces and Royal Marines undertook the operation . Battle of the Bulge . After the capture of Walcheren came the last great German offensive of the war . In a repeat of their 1940 attack , German formations smashed through weak Allied lines in the Ardennes in Belgium . The Battle of the Bulge presented a command problem to General Eisenhower . It had sliced through US lines , leaving some American formations north and south of the new German salient . However , the headquarters of U . S . 12th Army Group lay to the south , and so Eisenhower decided to place American forces north of the " Bulge " salient under 21st Army Group . They , with the American 3rd Army under General George S . Patton , reduced the salient . After the battle , control of the U . S . 1st Army which had been placed under Field Marshal Montgomery ' s temporary command was returned to Bradley ' s 12th Army Group . The U . S . 9th Army remained under Montgomery longer , before being returned to American command in Germany . Battle for the Roer Triangle . Prior to the Rhineland Campaign the enemy had to be cleared from the Roer Triangle during Operation Blackcock . This large methodical mopping up operation took place between 14 and 27 January 1945 . It was not planned to make any deep thrust into the enemy defences or capture large numbers of prisoners . It proceeded from stage to stage almost entirely as planned and was completed with minimal casualties . Rhineland Campaign . Allied forces closed up to the Rhine by March 1945 . Twenty - First Army Group at this time comprised the British Second Army under General Miles C Dempsey , the First Canadian Army under General Harry Crerar and the US Ninth Army , under General William Simpson . The First Canadian Army had executed Operation Veritable in difficult conditions from Nijmegen eastwards through the Reichswald Forest then southwards . This was to have been the northern part of a pincer movement with the US Ninth Army moving northwards towards Düsseldorf and Krefeld ( Operation Grenade ) , to clear the west bank of the Rhine north of Cologne . However the Americans were delayed by two weeks when the Germans destroyed the Roer dams and flooded the American route of advance . As a result the Canadians engaged and mauled the German reserves intended to defend the Cologne Plain . In Operation Plunder , starting on 13 March 1945 , the British 2nd Army and the US 9th Army crossed the Rhine at various places north of the Ruhr and German resistance in the west quickly crumbled . The First Canadian Army wheeled left and liberated the northern part of the Netherlands and adjoining areas of Germany , the British 2nd Army occupied much of north - west Germany and the US 9th Army formed the northern arm of the envelopment of German forces in the Ruhr Pocket and on 4 April reverted to Bradley ' s 12th Army Group . On 4 May 1945 , Field Marshal Montgomery accepted the unconditional surrender of the German forces in the Netherlands , in north west Germany and Denmark . British Army Of the Rhine . After the German surrender , 21st Army Group was converted into the headquarters for the British zone of occupation in Germany . It was renamed the British Army of the Rhine ( BAOR ) on 25 August 1945 and eventually formed the nucleus of the British forces stationed in Germany throughout the Cold War . Order of battle . The main constituent formations of 21st Army Group were the First Canadian Army and the British 2nd Army . In practice , neither of the two armies were homogeneously British or Canadian . Also included were Polish units , from Normandy onwards and small Dutch , Belgian , and Czech units ; American Army forces were attached from time to time . Lines of communications units were predominantly British . Attached U . S . units . American army units were placed under British command at various times . For political and personal rivalry reasons , these were never more than temporary . These arrangements occurred when