In May, before Cascade had become fully operational, Rommel defeated the British Army in Libya. Auchinleck rallied his forces at El Alamein and asked Clarke to draft plans for delaying the Axis advance, giving the Allies time to withdraw. Operation Sentinel was designed to convince Rommel, using camouflage, fakery and radio deception, that substantial British armour sat between him and the retreating Eighth Army. In spite of his success at El Alamein Auchinleck was replaced by Harold Alexander on , Clarke's third commander in under two years. Bernard Montgomery was handed control of the Eighth Army and instructed to push Rommel back. Montgomery knew Clarke, having taught him infantry tactics at the Staff College in 1931, and instructed him to prepare deception plans for the Second Battle of El Alamein. The plan involved major camouflage activity, Operation Bertram, radio deception, Operation Canwell and a disinformation campaign, Operation Treatment. On 9 September Clarke was appointed an Officer of the Order of the British Empire (OBE). The citation recognised his efforts in setting up A-Force, but referred to its less clandestine MI9 escape and evasion work.