“It is entirely clear that there is only one way in which great wars can be permanently prevented, and that is the establishment of an international government with a monopoly of serious armed force.”

What Does God Want Us to Do About Russia? (1948)

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.

Share International Magazine

p, 125
War and Change in World Politics (1981)

1920s, Freedom and its Obligations (1924)

The Robert Heinlein Interview (1973)
Context: I would say that my position is not too far from that of Ayn Rand's; that I would like to see government reduced to no more than internal police and courts, external armed forces — with the other matters handled otherwise. I'm sick of the way the government sticks its nose into everything, now.

Waldersee in his diary, 15 October 1885, quoted in Scott A. Silverstone, From Hitler's Germany to Saddam's Iraq

Source: The Production of Security (1849), p. 34-35
Context: Everywhere, when societies originate, we see the strongest, most warlike races seizing the exclusive government of the society. Everywhere we see these races seizing a monopoly on security within certain more or less extensive boundaries, depending on their number and strength.And, this monopoly being, by its very nature, extraordinarily profitable, everywhere we see the races invested with the monopoly on security devoting themselves to bitter struggles, in order to add to the extent of their market, the number of their forced consumers, and hence the amount of their gains.War has been the necessary and inevitable consequence of the establishment of a monopoly on security.Another inevitable consequence has been that this monopoly has engendered all other monopolies.

2009, Nobel Prize acceptance speech (December 2009)