Montgomery was able to persuade Eisenhower to adopt his strategy of a single thrust to the Ruhr with Operation Market Garden in September 1944. It was uncharacteristic of Montgomery's battles: the offensive was strategically bold, but poorly planned. Montgomery either did not receive or ignored ULTRA intelligence which warned of the presence of German armoured units near the site of the attack. As a result, the operation failed with the destruction of the British 1st Airborne Division at the Battle of Arnhem and the loss of any hopes of invading Germany by the end of 1944.
Montgomery's preoccupation with the push to the Ruhr had also distracted him from the essential task of clearing the Scheldt during the capture of Antwerp; and so, after Arnhem, Montgomery's group was instructed to concentrate on doing this so that the port of Antwerp could be opened.
When the surprise attack on the Ardennes took place on 16 December 1944, starting the Battle of the Bulge, the front of the U.S. 12th Army Group was split, with the bulk of the U.S. First Army being on the northern shoulder of the German 'bulge'. The Army Group commander, General Omar Bradley, was located south of the penetration at Luxembourg and command of the U.S. First Army became problematic. Montgomery was the nearest commander on the ground and on 20 December, Eisenhower (who was in Versailles) transferred Courtney Hodges' U.S. First Army and William Simpson's U.S. Ninth Army to his 21st Army Group, despite Bradley's vehement objections on national grounds. Montgomery grasped the situation quickly, visiting all divisional, corps, and army field commanders himself and instituting his 'Phantom' network of liaison officers. He grouped the British XXX Corps as a strategic reserve behind the Meuse and reorganised the US defence of the northern shoulder, shortening and strengthening the line and ordering the evacuation of St Vith. The German commander of the 5th Panzer Army, Hasso von Manteuffel said:
Eisenhower had then wanted Montgomery to go on the offensive on 1 January to meet Patton's army that had started advancing from the south on 19 December and in doing so, trap the Germans. However, Montgomery refused to commit infantry he considered underprepared into a snowstorm and for a strategically unimportant piece of land. He did not launch the attack until 3 January, by which point the German forces had been able to escape. A large part of American military opinion thought that he should not have held back, though it was characteristic of him to use drawn-out preparations for his attack. After the battle the U.S. First Army was restored to the 12th Army Group; the U.S. Ninth Army remained under 21st Army Group until it crossed the Rhine.
Montgomery's 21st Army Group advanced to the Rhine with operations Veritable and Grenade in February 1945. A carefully-planned Rhine crossing occurred on 24 March. While successful it was weeks after the Americans had unexpectedly captured the Ludendorff Bridge at Remagen and crossed the river. Montgomery's river crossing was followed by the encirclement of the German Army Group B in the Ruhr. Initially Montgomery's role was to guard the flank of the American advance. This was altered, however, to forestall any chance of a Red Army advance into Denmark, and the 21st Army Group occupied Hamburg and Rostock and sealed off the Danish peninsula.
On 4 May 1945, on Lüneburg Heath, Montgomery accepted the surrender of German forces in northern Germany, Denmark and the Netherlands. This was done plainly in a tent without any ceremony. In the same year he was awarded the Order of the Elephant, the highest order in Denmark.
After the war Montgomery became the C-in-C of the British Forces of Occupation and the British member of the Allied Control Council. He was made 1st Viscount Montgomery of Alamein in 1946. He was Chief of the Imperial General Staff from 1946 until 1948, succeeding Alanbrooke, but was largely a failure as it required strategic and political skills he did not have. He rarely spoke with his fellow commanders-in-chiefs, and sent his deputy to their meetings. He clashed particularly with Arthur Tedder, who as Deputy Supreme Commander had wanted Montgomery sacked during the Battle of Normandy, and who was by now Chief of the Air Staff. When Montgomery's term of office expired, Prime Minister Clement Attlee appointed General (later Field-Marshal) William Slim as his successor; when Montgomery protested that he had already promised the job to his protegé General Crocker, a former corps commander from the 1944-5 campaign, Attlee is said to have given the memorable retort "Untell him".
Montgomery then became Chairman of the Western European Union's commanders-in-chief committee. Volume 3 of Nigel Hamilton's "Life of Montgomery of Alamein" gives a good account of the bickering between Montgomery and his land forces chief, a French general, which created splits through the Union headquarters. He was thus pleased to become Eisenhower's deputy in creating the North Atlantic Treaty Organisation's European forces in 1951. He was an effective inspector-general and mounted good exercises, but out of his depth politically, and his exacting manner and emphasis on efficiency created ill-feeling. He continued to serve under Eisenhower's successors, Matthew Ridgway and Al Gruenther, until his retirement, aged nearly 71, in 1958. His mother died in 1949; Montgomery did not attend the funeral, claiming he was "too busy". He was chairman of the governing body of St John's School, Leatherhead, Surrey from 1951 to 1966 and a generous supporter. Montgomery was an Honorary Member of the Winkle Club, a noted charity in Hastings, East Sussex, and introduced Winston Churchill to the club in 1955.
In 1953, the Hamilton Board of Education in Hamilton, Ontario, Canada, wrote to Montgomery and asked permission to name a new school in the city's east end after him. Viscount Montgomery Elementary was billed as "the most modern school in North America" and the largest single-storey school in Hamilton, when the sod was turned on 14 March 1951. The school officially opened on 18 April 1953, with Montgomery in attendance among almost 10,000 well-wishers. At the opening, he gave the motto "Gardez Bien" from his own family's coat of arms.
Montgomery referred to the school as his "beloved school" and visited on five separate occasions, the last being in 1960. On his last visit, he said to "his" students:
Before retirement, Montgomery's outspoken views on some subjects, such as race, were often officially suppressed. After retirement these outspoken views became public and his reputation suffered. He supported apartheid and Chinese communism under Mao Zedong, and spoke against the legalisation of homosexuality in the United Kingdom, arguing that the "Sexual Offences Act 1967" was a "charter for buggery" and that "this sort of thing may be tolerated by the French, but we're British — thank God." However, several of Montgomery's biographers, including Chalfont (who found something "disturbingly equivocal" in "his relations with boys and young men" ) and Nigel Hamilton (2002) have suggested that he may himself have been a repressed homosexual; in the late 1940s he conducted an affectionate friendship with a 12-year-old Swiss boy.