The quote "In peace, prepare for war. In war, prepare for peace." is famous quote attributed to Sun Tzu (-543–-495 BC), ancient Chinese military general, strategist and philosopher from the Zhou Dynasty.
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
“The only way to have peace is to prepare for war.”
William Pitt, 1st Earl of Chatham (1708–1778) British politician
To the Duke of Newcastle (c. August 1758), quoted in Basil Williams, The Life of William Pitt, Earl of Chatham. Volume I (London: Longmans, 1913), p. 379
1750s
“Happy the state which in times of peace is yet prepared for war.”
Felix est illa civitas quae in pace bellum cogitat.
Gerald of Wales (1146) Medieval clergyman and historian
Book 2, chapter 9, p. 271.
Compare Vegetius De Re Militari: "Qui desiderat pacem, praeparet bellum" (Let him who desires peace prepare for war).
Descriptio Cambriae (The Description of Wales) (1194)
“To be prepared for war is one of the most effectual means of preserving peace.”
George Washington (1732–1799) first President of the United States
First Annual Address, to both Houses of Congress (8 January 1790).
Compare: "Qui desiderat pacem præparet bellum" (translated: "Who would desire peace should be prepared for war"), Vegetius, Rei Militari 3, Prolog.; "In pace, ut sapiens, aptarit idonea bello" (translated: "In peace, as a wise man, he should make suitable preparation for war"), Horace, Book ii. satire ii.
1790s
“In peace, as a wise man, he should make suitable preparation for war.”
in pace, ut sapiens, aptarit idonea bello
Book II, satire ii, line 111
Satires (c. 35 BC and 30 BC)
Frederick B. Maurice (1871–1951) British Army general and historian
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.
“We must be prepared to pay the price for peace, or assuredly we shall pay the price of war.”
Harry Truman (1884–1972) American politician, 33rd president of the United States (in office from 1945 to 1953)
Special Message to the Congress on the Threat to the Freedom of Europe (1948)
Context: The recommendations I have made represent the most urgent steps toward securing the peace and preventing war. We must be ready to take every wise and necessary step to carry out this great purpose. This will require assistance to other nations. It will require an adequate and balanced military strength. We must be prepared to pay the price for peace, or assuredly we shall pay the price of war. We in the United States remain determined to seek peace by every possible means, a just and honorable basis for the settlement of international issues.
Robert Aumann (1930) Israeli-American mathematician
Quoted by Elad Benari in The Frantic Desire for Peace Only Brings War.
Source: http://www.israelnationalnews.com/News/News.aspx/140029#.VDs8U_ldUa4
