Slessor was an RAF Commander who played a major role in the battle against the German U-Boats. When war broke out he was director of the Plans Branch of the Air Ministry. In 1940 he participated in a conference of US and British Army, Navy and Air Force officers in the United States. The conference confirmed the ‘Germany first’ policy. Slessor then became Air Officer Commanding No 5 Bomber Group, RAF Bomber Command. In 1942 he was given the newly formed post of Assistant Chief of the Air Staff (Policy) and attended all the major Allied conferences. In 1943 Slessor became head of RAF Coastal Command, co-operating with the Royal Navy and the US forces in an intensive effort to defeat the U-Boats in the Battle of the Atlantic. The use of aircraft in this fight turned out to be crucial. Convoys escorted by planes were rarely attacked because they made it impossible for the German boats to operate in wolf-packs and air to sea rockets scored many hits.
Victory in this struggle was essential both to maintain the merchant fleet and to make an Allied landing in France possible. In January 1944 Slessor became Commander in Chief of RAF units in the Mediterranean and Deputy Commander in Chief of Allied Air Forces in that theater. In his capacity as Deputy Commander in Chief he was involved in the Riviera landings of August 1944.