Slessor held several operational commands in the Second World War. As Air Officer Commanding Coastal Command in 1943 and 1944, he was credited with doing much to turn the tide of the Battle of the Atlantic through his use of long-range bombers against German U-boats. He was knighted in June 1943. In the closing stages of the war he became Commander-in-Chief RAF Mediterranean and Middle East and deputy to Lieutenant General Ira Eaker as Commander-in-Chief Mediterranean Allied Air Forces, conducting operations in the Italian Campaign and Yugoslavia. Slessor went on to serve in the RAF's most senior post, Chief of the Air Staff, in the early 1950s, and was considered a strong proponent of strategic bombing and the nuclear deterrent. In retirement he published two more books, including an autobiography, and held ceremonial appointments in Somerset.