Schlacht im Reichswald
Cassels served in the Second World War as commander of the 152nd Infantry Brigade, commanding the brigade during Operation Goodwood, Operation Totalize and Operation Veritable, before becoming General Officer Commanding 51st (Highland) Division in the closing stages of the war. He later commanded the 1st Commonwealth Division in the Korean War and was General Officer Commanding of the 1 (British) Corps before becoming director of operations in Malaya during the Malayan Emergency.