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.
