On 18 September 1939, shortly after the beginning of the Second World War, Ellis returned to the Welsh Guards[REF] and was later appointed as an official historian, contributing three volumes to the History of the Second World War.
His The War in France and Flanders (1954) begins with the Phoney War of 1939-1940 and deals with the failed attempts of the British Expeditionary Force to defend Belgium and France from the German invasion of May and June 1940.