Gregory S. "Greg" Newbold is a retired lieutenant general in the United States Marine Corps, whose last assignment was as Director of Operations for the Joint Staff. He retired, in part, due to his opposition to the planned Iraq War. 
Prior to the Joint Staff, he commanded the 1st Marine Division, and had headed Marine infantry units at platoon, company and battalion levels.  He commanded the 15th Marine Expeditionary Unit during the initial Operation RESTORE HOPE deployment in Somalia. LTG Newbold held national-level staff posts in the Joint Staff, Office of the Secretary of the Navy, and Marine Corps headquarters.
Newbold had been in Britain, discussing Operation NORTHERN WATCH and Operation SOUTHERN WATCH, when the 9-11 attack struck. On his return, he said Douglas Feith asked him "why are you working on Afghanistan? You ought to be working on Iraq." Feith denied this, saying he had been conveying Rumsfeld's thinking about considering the war on terrorism in a broader context. 
After the 9-11 attack, in late December 2001, Secretary of Defense Donald Rumsfeld had OPLAN 1003-98 presented by LTG Greg Newbold, director of operations for the Joint Chiefs of Staff, in late 2001. Rumsfeld believed the plan, which called for up to 500,000 troops, was far too large; Rumsfeld thought that no more than 125,000 would be needed. Newbold later said he regretted he did not say, at the time,
Mr. Secretary, if you try to put a number on a mission like this, you may cause enormous mistakes. Give the military the task, give the military what you would like to see them do, and let them come up with it. I was the junior military man in the room, but I regret not saying it
He was especially critical of Douglas Feith and his staff, whom he said "were essentially an extra-governmental organization, because many of their sources of information and much of their work was in the shadows. It was also my sense that they cherry-picked obscure, unconfirmed information to reinforce their own philosophies and ideologies."
He was later among a group of retired generals who objected to the Administration policy. A number of retired generals have been highly critical of the plan, focused especially on what they considered the unrealistic goals of Secretary of Defense Donald Rumsfeld. Besides Newbold, they include Paul Eaton, who headed training of the Iraqi military in 2003-2004; formal chiefs of United States Central Command (Anthony Zinni and Joseph Hoar); John Riggs, a planner who had criticized personnel levels, in public, while on duty; division commanders Charles Swannack and John Batiste.
- Greg Newbold (April 09, 2006), "Why Iraq Was a Mistake", Time
- Lieutenant General Gregory S. Newbold (retired), Headquarters, United States Marine Corps
- Michael R. Gordon, Bernard E. Trainor (2006), COBRA II: the inside story of the invasion and occupation of Iraq, Pantheon, ISBN 0375422625, p. 15
- COBRA II, p. 4
- Thomas E. Ricks (2006), FIASCO: the American Military Adventure in Iraq, Penguin, ISBN 159320103X, p. 55
- Deary, David S. (February 23, 2007), Six agaist the Secretary: the Retired Generals and Donald Rumsfeld, Air War College