Archibald James Halkett Cassels, (1907-1996), was the son of General Sir Robert Cassels, and was educated at Rugby and Sandhurst and was commissioned into the Seaforth Highlanders in 1926. In 1930 he served with the 2nd Battalion at Jhansi and then took part in operations in the North West Frontier province, on the Khajauri Plain. At the end of 1931 he moved with the battalion to Haifa, becoming adjutant soon afterwards, and was involved in the Arab disturbances in 1933. The next year he moved with the regiment back to Dover, but returned to India in the late 1930s to become ADC to his father, who was commander-in-chief at the time. In France with the British Expeditionary Force, Cassels served with 157 Brigade and was then brigade major to Brigadier Sir John Laurie on the withdrawal to St Valéry en Caux in 1940. His regiment fought extremely well in the long retreat, often opposing German tanks with small-arms fire, and resisted capitulation at St Valéry until French troops carrying white flags marched across their front, masking their guns.Soon after the Normandy landings, in June 1944, Cassels took over command of 152 Infantry Brigade (51st Highland Division) on the beachhead east of the river Orne. 51st Highland Division was a reconstituted unit, bearing the name of its predecessor which had been taken prisoner at St Valéry four years earlier. Cassels led 152 Infantry Brigade with distinction through the capture of Le Havre, the winter operations in Holland, the Ardennes battles and the final advance into Germany. In 1945 he was appointed General Officer Commanding 51st Highland Division in the Bremen district. By the end of the war Cassels had been mentioned in despatches twice and, in 1944, awarded a D.S.O.

In 1951 he became G.O.C., 1st British Commonwealth Division in Korea. The division, composed of Canadian, Australian, New Zealander and British troops, achieved a first-class reputation, due in no small part to the personality, diplomacy and judgment of Cassels. He was awarded the US Legion of Merit. From 1953 to 1954 he was Commander of 1 (British) Corps in Germany, after which he returned to Britain to become Director-General of Military Training at the War Office for the next three years. Then followed two years as Director of Emergency Operations in the Federation of Malaya. In 1959 Cassels returned to England to become G.O.C-in-C, Eastern Command, and then Commander-in-Chief, British Army of the Rhine, and Commander (NATO) Northern Army Group, from 1960 to 1963. From 1963 to 1964 he was Adjutant General, and then Chief of the General Staff from 1965 to 1968. He was promoted Field Marshal in 1968.

From 1957 to 1961, Cassels was Colonel, Seaforth Highlanders.

According to his Daily Telegraph obituary, the young Cassels was also ‘an enthusiast for what used to be called "dance music", and sang the latest hits to his own accompaniment on guitar - something that nobody of lesser calibre could have got away with in the still-Edwardian mess atmosphere of the 1930s.’