The term 'Chief of ___' can be filled in with the word 'Haig, ' which is a four-letter word. The term 'Haig' is a reference to General Douglas Haig, who was a British Army officer during World War I. He was appointed as Commander-in-Chief of the British Expeditionary Force in 1915 and held this position until the end of the war in 1918. He was responsible for leading the British forces on the Western Front and was widely criticized for his tactics and strategies.