Question: William Corcoran Eustis (July 20, 1862 - November 24, 1921) was a captain in the United States Army and the personal assistant to which General of the Armies during World War I, who was was a senior United States Army officer, that had a nickname of "Black Jack"?
Sentence (from "John J. Pershing"): His most famous post was when he served as the commander of the American Expeditionary Force (AEF) on the Western Front in World War I, 1917–18.
Relevant evidence: No