From 12 September 1942 until December 1945 Adair was General Officer Commanding (GOC) of the Guards Armoured Division, taking over from Oliver Leese, receiving promotion to colonel on 30 June 1943, while serving as an acting and then temporary major general from 21 September 1942. After training the division throughout the United Kingdom for 21 months, the Guards Armoured Division arrived in Normandy as part of Operation Overlord on 28 June 1944 as part of Lieutenant General Sir Richard O'Connor's VIII Corps, first seeing action during Operation Goodwood in July, and then in Operation Bluecoat in July/August. Following the Allied break-out from Normandy they advanced across Northern France and into Belgium as part of Lieutenant General Sir Brian Horrocks' XXX Corps. The division liberated Brussels, after making an unprecedented advance from Douai, away, in only 14 hours. The division then took a leading role in the ground advance in Operation Market Garden in September. Held in reserve during the Battle of the Bulge in December, it was committed to the Battle of the Reichswald (Operation Veritable) in February and March 1945. After advancing through Germany and the German surrender in May 1945 the Guards Armoured Division remained as part of the occupying forces, but on 12 June 1945 was converted into an infantry formation, the Guards Division.