“You cannot simultaneously prevent and prepare for war.”

Speaking in Carnegie Hall, New York City, on 4 April 1919.

Context: As a soldier who has spent a quarter of his life in the study of the science of arms, let me tell you I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare thoroughly and efficiently for war, you get war.

Sometimes erroneously prepended to the opening line "The art of war is of vital importance to the State", but appears to be a variation of the Roman motto "Si vis pacem, para bellum". It's not clear who first misattributed this phrase to Sun Tzu. The earliest appearance of the phrase in Google Books is 1920, when it appeared in a pharmaceutical journal, but no attribution was given then.
Misattributed

Source: Letter to his daughter (1978), p. 15
Context: You cannot be big unless you are prepared to kiss the ground. You cannot defend the soil unless you know the smell of that soil. I know the smell of our soil. I know the rhythm of our rivers. I know the beat of our drums. The theories, the dogmas and the scripts stand outside the gates of history. The dominant factor is the aspiration of the people and the ability to seek total identification with it. Once the significance of the symphony is grasped, the lines fall into place, the dogmas and theories get legs to move in time to the majesty of that music. This does not mean that I am preaching pragmatism. There is a lot of expediency in pragmatism. I am trying to trace the roots of the problems, the genesis of the challenges, the cause of the struggle.

Source: The Wisdom of W.E.B. Du Bois (2003), p. 75

Source: The Tales of Alvin Maker, The Crystal City (2003), Chapter 9 “Expeditions” (p. 166).

Source: Memory, Sorrow, and Thorn, The Dragonbone Chair (1988), Chapter 34, “Forgotten Swords” (p. 549).

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

Speaking in Carnegie Hall, New York City, on 4 April 1919.
[New York Times, 5 April 1919, 13, Maurice Criticises Peace Conferees, http://query.nytimes.com/gst/abstract.html?res=9B00E6DC1E3BEE3ABC4D53DFB2668382609EDE]

Revolutionary
Albums, Revolutionary Vol. 1 (2001)