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"): General of the Armies John Joseph "Black Jack" Pershing (September 13, 1860 – July 15, 1948) was a senior United States Army officer.
Relevant evidence: Yes