“Wars can be prevented just as surely as they can be provoked, and we who fail to prevent them must share the guilt for the dead.”

Hope, Despair, and Memory (1986)

Source: https://clusterbcs.com/en/in-memoriam-kirk-r-smith-1947-2020-2/

Various sources below attribute this statement or similar ones to Marshall
But a war to prevent a third world war would be the Third World War, and Marshall had reached the conclusion that, "The only way human beings can win a war is to prevent it."
As quoted in This is Our World (1956) by Louis Fisher, p. 91
Marshall's motto read: "The only way human beings can win a war is to prevent it." It was 1947.
As quoted in The Story of Indonesia (1959) by Louis Fisher, p. 111 http://books.google.de/books?id=AkIeAAAAMAAJ&q=motto+read
Frances Perkins recalled his saying, "The only way human beings can win a war is to prevent it."
As quoted in Freedom's Advocate: a twenty-five year chronicle (1965) by Aaron Levenstein, p. 104 http://books.google.de/books?id=plZIAQAAIAAJ&q=perkins
“Its purpose is to avoid war, not to provoke it,” he explained to his goddaughter, Rose Page Wilson. The deterrence factor was vital. “The only way to be sure of winning a third world war is to prevent it,” Marshall warned.
As quoted in General of the Army. George C. Marshall, Soldier and Statesman by Ed Cray (1990) p. 645 http://books.google.de/books?id=bGgcYteOQxUC&pg=PA645
Unsourced variant: The only way to win a war is to prevent it.
A very similar statement appears in the US Strategic Bombing Survey Summary Report (European War) (30 September 1945), p. 41 http://books.google.de/books?id=mnChmcVKoVsC&pg=PA41&dq=lesson:
:: The great lesson to be learned in the battered towns of England and the ruined cities of Germany is that the best way to win a war is to prevent it from occurring.

Hope, Despair, and Memory (1986)

2012, Sandy Hook Prayer Vigil (December 2012)
Context: Can we honestly say that we’re doing enough to keep our children, all of them, safe from harm?
Can we claim, as a nation, that we’re all together there, letting them know they are loved and teaching them to love in return?
Can we say that we’re truly doing enough to give all the children of this country the chance they deserve to live out their lives in happiness and with purpose?
I’ve been reflecting on this the last few days, and if we’re honest with ourselves, the answer’s no. We’re not doing enough. And we will have to change. Since I’ve been president, this is the fourth time we have come together to comfort a grieving community torn apart by mass shootings, fourth time we’ve hugged survivors, the fourth time we’ve consoled the families of victims.
And in between, there have been an endless series of deadly shootings across the country, almost daily reports of victims, many of them children, in small towns and in big cities all across America, victims whose — much of the time their only fault was being at the wrong place at the wrong time.
We can’t tolerate this anymore. These tragedies must end. And to end them, we must change. We will be told that the causes of such violence are complex, and that is true. No single law — no set of laws can eliminate evil from the world, or prevent every senseless act of violence in our society. But that can’t be an excuse for inaction. Surely, we can do better than this.
If there is even one step we can take to save another child, or another parent, or another town, from the grief that has visited Tucson, and Aurora, and Oak Creek, and Newtown, and communities from Columbine to Blacksburg before that — then surely we have an obligation to try.

To Leon Goldensohn, March 10, 1946, "The Nuremberg Interviews" by Leon Goldensohn - History - 2007

Letter to Guy Fleetwood Wilson (13 November 1936), quoted in Martin Gilbert, Prophet of Truth: Winston S. Churchill, 1922–1939 (London: Minerva, 1990), p. 800
The 1930s

Interview, 18 March 2002