Cassels served in the Second World War, where he was serving in England, and he attended a shortened course at the Staff College, Camberley. In May 1940 he then became a brigade major with a Territorial Army (TA) unit, the 157th Infantry Brigade, commanded by Brigadier Sir John Laurie, formerly Cassels's Commanding Officer (CO) in the 2nd Seaforths, part of Major General James Drew's 52nd (Lowland) Infantry Division, which was sent to France in June 1940 to bolster the French Army in the aftermath of the British Expeditionary Force's (BEF) evacuation from Dunkirk. Cassel's brigade, the first of the 52nd Division's brigades to arrive in France, was, soon after its arrival, ordered to form the right of the French Tenth Army at Conches, Normandy. However, the situation was considered to be hopeless and Cassel's brigade was later withdrawn through Cherbourg on 18 June. The French signed the second armistice of Compiègne just days later.