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 "Lucian Truscott"): He was, along with Alexander Patch and James Van Fleet, among the few U.S. Army officers to command a division, a corps, and a field army on active service during the war.
Relevant evidence: No