“Huns should only enter wars in which they can win.”

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.

Speech, reported in Robert G. Torricelli, Andrew Carroll, In Our Own Words: Extraordinary Speeches of the American Century (2000), p. 126.

On the war against terrorism
Have We Already Been Defeated? (2001)
Context: Power can win the body count but it cannot win this war. Because the enemy is not human. This is a war against a malicious spirit. Only fools attempt to defeat a spirit with guns and rockets and bombs.

Freedom is a Constant Struggle: Closures and Continuities (2013)

Address at a Rally in the Civic Auditorium, Seattle, Washington (October 17, 1956). Source: Eisenhower Presidential Library. Archived https://web.archive.org/web/20210125121539/https://www.eisenhowerlibrary.gov/eisenhowers/quotes from the original https://www.eisenhowerlibrary.gov/eisenhowers/quotes on January 25, 2021.
1950s

For All We Have and Are http://whitewolf.newcastle.edu.au/words/authors/K/KiplingRudyard/verse/p1/forall.html, Stanza 1 (1914).
Other works

First Inaugural Address (March 4, 1897).
1890s

Guerrilla Warfare (1960)