With the United States' entry into World War I, Vaughan was commissioned second lieutenant in the Field Artillery and was assigned for military training at Fort Sill, Oklahoma.

He served as a liaison officer and battery commander, and for his service in combat, he was later decorated with two Silver Stars and the French Croix de Guerre.

[2] Vaughan returned to active duty in World War II, was injured in a plane crash in 1943, and was assigned to the staff of the Truman Committee.

[3] When Truman was vice president, Vaughan went to Treasury Secretary Henry Morgenthau and demanded a Secret Service agent be assigned to him.

In 1951, White House Appointments Secretary Matthew J. Connelly asked legal counsel Max Lowenthal to help General Harry H. Vaughan in "setting up testimony.