Answer by taking a quote from the following article:

General Sir Miles Christopher Dempsey,  (15 December 1896 - 5 June 1969) was a senior British Army officer who served in both world wars. As a junior officer, he fought in France during the First World War, where he was wounded, and served throughout the difficult interwar period, travelling to various corners of the globe. During the Second World War he had a close relationship with Bernard Montgomery and commanded the 13th Brigade in France in 1940, and spent the next two years training troops in England, before commanding XIII Corps for the invasions of Sicily and Italy in 1943. He later commanded the Second Army during the Battle of Normandy and made rapid advances in the subsequent campaign in Northern France and Belgium and was the first British Army commander to cross the Rhine.

In North Africa, Sicily and Italy, Montgomery's faith in Dempsey had proved justified and he had also gained a reputation for his expertise in Combined Operations. This prompted Montgomery, when he left Italy at the end of 1943 to take command of the 21st Army Group for the forthcoming D-Day landings, to select Dempsey to command the British Second Army. The Second Army was the main British force (although it also included Canadian Army units) involved in the landings, making successful assaults at Gold, Juno and Sword beaches on 6 June 1944.  The successful assaults were followed by a battle of attrition during which the Anglo-Canadian forces were frustrated by determined German resistance. This fighting drew vital German units including the bulk of their armoured strength to the Caen sector, facilitating the breakout further west in July (see Operation Cobra) by Lieutenant General George S. Patton's U.S. Third Army. The Second Army then made a rapid advance across northern France into Belgium, liberating Brussels and Antwerp in September 1944. On 15 October 1944, during a visit to the Second Army, King George VI knighted Dempsey on the battlefield. Because of the fast and successful advance over more than 200 miles in a week Dempsey received the nickname of "Two Hundred Miles" Dempsey.  Second Army's XXX Corps (now commanded by Lieutenant General Brian Horrocks) took part in Operation Market Garden, the failed attempt to secure an early crossing of the River Rhine in September 1944, which Dempsey believed could not succeed and openly questioned to Montgomery. Airborne troops secured a succession of canal and river crossings to enable XXX Corps to reach the Lower Rhine at Arnhem and wheel right into Germany. Intelligence had not detected the presence of unexpected German formations in the area and resistance proved greater than expected frustrating XXX Corps attempts to reach its final objective. During the operation, Dempsey, forward near the front with his Tac HQ, witnessed the crossing of the U.S. 82nd Airborne Division's 504th Parachute Regiment cross the Nijmegen bridge. Impressed, he later wrote that the 82nd was "easily the best division on the Western front". Dempsey met with the 82nd's commander, Brigadier General James M. Gavin, shook him by the hand and said "I am proud to meet the commander of the greatest division in the world today."  The Second Army, with XII and XXX Corps in the vanguard and II Canadian Corps under command and VIII Corps in reserve eventually crossed the Rhine on 23 March 1945, and Dempsey was the first British Army commander to do so. On 7 April 1945, The Illustrated London News carried a full front page of a specially commissioned portrait painting of Dempsey by artist Arthur Pan. In May, Dempsey's men captured Bremen, Hamburg and Kiel. At 11.00 am on 3 May, a delegation of senior German officers led by General Admiral von Friedeberg arrived at Dempsey's Tac HQ and after questioning it appeared that Friedeberg was a representative of Generalfeldmarschall Keitel and Grand Admiral Donitz who wished to surrender. In typical fashion Dempsey sent them on their way to report to Montgomery which led to the formal surrender the next day at Luneberg Heath.

what happened in 1944?
involved in the landings, making successful assaults at Gold, Juno and Sword beaches on 6 June 1944.