“If saving human lives is the great desideratum, then there is more to be gained by the prevention of drowning, and auto wrecks than by the abolition of war.”

"The Gentle Vultures" in Super-Science Fiction (December 1957)
General sources

Letter to Robert Morris https://web.archive.org/web/20060503040039/http://gwpapers.virginia.edu/project/volumes/confederation/essay4.html (12 April 1786)
1780s

Source: Styxx

Nuremberg trials, (31 August 1945)

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.

Principles of Social Reconstruction [Originally titled Why Men Fight : A Method Of Abolishing The International Duel], Ch. VIII : What We Can Do, p. 257
1910s
Context: It is preoccupation with possessions, more than anything else, that prevents men from living freely and nobly. The State and Property are the great embodiments of possessiveness; it is for this reason that they are against life, and that they issue in war. Possession means taking or keeping some good thing which another is prevented from enjoying; creation means putting into the world a good thing which otherwise no one would be able to enjoy. Since the material goods of the world must be divided among the population, and since some men are by nature brigands, there must be defensive possession, which will be regulated, in a good community, by some principle of impersonal justice. But all this is only the preface to a good life or good political institutions, in which creation will altogether outweigh possession, and distributive justice will exist as an uninteresting matter of course.
The supreme principle, both in politics and in private life, should be to promote all that is creative, and so to diminish the impulses and desires that center round possession.

Source: Being Peace

1870s, Oratory in Memory of Abraham Lincoln (1876)
Context: Fellow citizens, whatever else in this world may be partial, unjust, and uncertain, time, time! is impartial, just, and certain in its action. In the realm of mind, as well as in the realm of matter, it is a great worker, and often works wonders. The honest and comprehensive statesman, clearly discerning the needs of his country, and earnestly endeavoring to do his whole duty, though covered and blistered with reproaches, may safely leave his course to the silent judgment of time. Few great public men have ever been the victims of fiercer denunciation than Abraham Lincoln was during his administration. He was often wounded in the house of his friends. Reproaches came thick and fast upon him from within and from without, and from opposite quarters. He was assailed by Abolitionists; he was assailed by slave-holders; he was assailed by the men who were for peace at any price; he was assailed by those who were for a more vigorous prosecution of the war; he was assailed for not making the war an abolition war; and he was bitterly assailed for making the war an abolition war. But now behold the change. The judgment of the present hour is, that taking him for all in all, measuring the tremendous magnitude of the work before him, considering the necessary means to ends, and surveying the end from the beginning, infinite wisdom has seldom sent any man into the world better fitted for his mission than Abraham Lincoln. His birth, his training, and his natural endowments, both mental and physical, were strongly in his favor. Born and reared among the lowly, a stranger to wealth and luxury, compelled to grapple single-handed with the flintiest hardships of life, from tender youth to sturdy manhood, he grew strong in the manly and heroic qualities demanded by the great mission to which he was called by the votes of his countrymen. The hard condition of his early life, which would have depressed and broken down weaker men, only gave greater life, vigor, and buoyancy to the heroic spirit of Abraham Lincoln. He was ready for any kind and any quality of work. What other young men dreaded in the shape of toil, he took hold of with the utmost cheerfulness.