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"?

Context articles: J. Lawton Collins
Henry W. Butner
John W. O'Daniel
Wade H. Haislip
Alexander Patch
Charles W. Bagnal
Lucian Truscott
William Corcoran Eustis
John J. Pershing
Charles L. Bolte

General Joseph "Lightning Joe" Lawton Collins (May 1, 1896 – September 12, 1987) was a senior United States Army officer who served in World War II and became Chief of Staff of the United States Army during the Korean War.
 During World War II, he served in both the Pacific and European Theaters of Operations (ETO), one of few senior American commanders to do so.
Henry Wolfe Butner (April 6, 1875 – March 13, 1937) was a United States Army general in World War I and onetime commanding officer of Fort Bragg (1928–29).
 A native of North Carolina, Butner graduated in the top half of the United States Military Academy Class of 1898.
 He became an artillery officer and was sent to France with the American Expeditionary Force during World War I.
 In the last month of the war Butner commanded an artillery brigade after promotion to brigadier general.
 After attending the United States Army War College, he led the United States Army Field Artillery School, the 24th Field Artillery Regiment, Fort Bragg, and Fort Eustis.
 Promoted to major general in eatly 1936, Butner took command of the Panama Canal Department.
 He suffered a stroke while golfing in late 1936, and died at Walter Reed Army Hospital in March 1937.
Lieutenant General John Wilson O'Daniel (February 15, 1894 – March 27, 1975), nicknamed "Iron Mike", was a senior United States Army officer who served in World War I, World War II, and the Korean War.
 He is perhaps best known for serving with the 3rd Infantry Division in North Africa, Sicily, Italy, and Southern France during World War II.
 He was the commanding general (CG) of Audie Murphy, the most decorated soldier in the U.S. Army during World War II.
General Wade Hampton Haislip (July 9, 1889 – December 23, 1971) was a senior United States Army officer who served in both World War I and World War II, where he led the XV Corps in the campaign in Western Europe from 1944 to 1945.
 He later became a four-star general, serving as Vice Chief of Staff of the United States Army (VCSA) from 1949 to 1951.
General Alexander McCarrell "Sandy" Patch (November 23, 1889 – November 21, 1945) was a senior United States Army officer, who fought in both World War I and World War II.
 During World War II he commanded U.S. Army and U.S. Marine Corps forces during the Guadalcanal Campaign, and the U.S. Seventh Army on the Western Front.
 He died in November 1945, a few months after the end of the war and two days before his 56th birthday.
 Patch was, along with James Van Fleet and Lucian K. Truscott, Jr., one of the very few senior American commanders to command a division, corps and field army on active service during World War II.
Lieutenant General Charles Wilson Bagnal (April 15, 1934 – June 30, 2015) was a United States Army officer.
 He was commander of the United States Army Western Command (later United States Army Pacific), from 1985 to 1989.
 Previously he was Deputy Commanding General for Training of the United States Army Training and Doctrine Command (TRADOC), Deputy Superintendent at the United States Military Academy (from 1977 to 1980), Commander of the 101st Airborne Division (1981-1983), Commander of the Officer Personnel Management Directorate for the United States Army Military Personnel Center, and Special Assistant to the Deputy Chief of Staff for Personnel.
 He is an alumnus of the United States Military Academy, United States Army Command & General Staff College, Georgia Tech, the United States Army War College and McLenaghan High School in Florence, South Carolina (class of 1952).
 He retired August 31, 1989, and later obtained his juris doctor from the University of South Carolina and practiced law.
 He resided in Columbia with his wife Patsy.
 Bagnal died on June 30, 2015 after a battle with leukemia.
General Lucian King Truscott Jr. (January 9, 1895 – September 12, 1965) was a highly decorated senior United States Army officer, who saw distinguished active service during World War II.
 Between 1943–45, he successively commanded the 3rd Infantry Division, VI Corps, and, Third United States Army.
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.
William Corcoran Eustis (July 20, 1862 - November 24, 1921) was a captain in the United States Army and the personal assistant to General John J. Pershing during World War I.
 He was chairman of the inauguration committee for the first inauguration of Woodrow Wilson in 1913 and started the Loudoun Hunt in 1894.
General of the Armies John Joseph "Black Jack" Pershing (September 13, 1860 – July 15, 1948) was a senior United States Army officer.
 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.
General Charles Lawrence Bolte (May 8, 1895 – February 11, 1989) was a senior United States Army officer who fought in both World War I and World War II.
 In World War II he distinguished himself as commander of the 34th Infantry Division during the Italian Campaign, for which he was twice awarded the Army Distinguished Service Medal.
 Later promoted to four-star general officer rank, his final post was Vice Chief of Staff of the United States Army.