During his first tour in Vietnam (1967–1968), then Colonel McDonough commanded the 2nd Brigade, 1st Air Cavalry Division which played a crucial role in Operation Pegasus, the relief of the besieged U.S. Marine Combat Base at Khe Sanh in April 1968.

[2] He graduated from the United States Military Academy at West Point in June 1945 with a Bachelor of Science degree and was commissioned a second lieutenant in the Infantry.

Upon the completion of parachute training in 1948, McDonough joined the 82nd Airborne Division at Fort Bragg, North Carolina where he served for three years as a company grade commander and staff officer.

[1] McDonough returned from Korea in 1953 and attended the Army Command and General Staff College at Fort Leavenworth, Kansas.

He reported for duty to the United States Naval Academy at Annapolis, Maryland one year later and served there as an instructor until November 1958.

[1] From January 1959 to May 1960, he served in the Plans Branch, Assistant Chief of Staff, Intelligence, Headquarters, U.S. Army, Europe in Heidelberg, West Germany.

During this period the battalion participated in Operation Pershing I and II which was a battle against enemy forces and an effort to assist the Vietnamese in establishing a pacification program in Bình Định province.

The 1st Cavalry Division completed the relief of the Marine units on 10 April and assumed responsibility for the Khe Sanh area of operations.