What Does God Want Us to Do About Russia? (1948)
Context: Instead of spending our money on preparedness to fight, we should pour out billions in a common effort to solve common problems around the earth... on the scale of present preparations for total war.... This vast sum should be spent on food, clothing, shelter, medicine, seed, fertilizer, livestock, machinery, tools and other requirements of efficient production and distribution.... Instead of spending billions on preparedness to wage war with pestilence, vast sums should be spent on health measures and the eradication of plague from the face of the earth.
“Therefore, we should devote ourselves as a preparative to preparedness, alike in peace and war, to secure the three elemental things:”
1910s, Address to the Knights of Columbus (1915)
Context: Therefore, we should devote ourselves as a preparative to preparedness, alike in peace and war, to secure the three elemental things: one, a common language, the English language; two, the increase in our social loyalty citizenship absolutely undivided, a citizenship which acknowledges no flag except the flag of the United States and which emphatically repudiates all duality of intention or national loyalty; and third, an intelligent and resolute effort for the removal of industrial and social unrest, an effort which shall aim equally at securing every man his rights and to make every man understand that unless he in good faith performs his duties he is not entitled to any rights at all.
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Theodore Roosevelt 445
American politician, 26th president of the United States 1858–1919Related quotes

“In peace, prepare for war. In war, prepare for peace.”
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

1910s, Address to the Knights of Columbus (1915)
Context: There must be not merely preparedness in things material; there must be preparedness in soul and mind. To prepare a great army and navy without preparing a proper national spirit would avail nothing. And if there is not only a proper national spirit, but proper national intelligence, we shall realize that even from the standpoint of the army and navy some civil preparedness is indispensable. For example, a plan for national defense which does not include the most far-reaching use and cooperation of our railroads must prove largely futile. These railroads are organized in time of peace. But we must have the most carefully thought out organization from the national and centralized standpoint in order to use them in time of war. This means first that those in charge of them from the highest to the lowest must understand their duty in time of war, must be permeated with the spirit of genuine patriotism; and second, that they and we shall understand that efficiency is as essential as patriotism; one is useless without the other.

“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)

“He, therefore, who desires peace, should prepare for war. He who aspires to victory, should spare no pains to form his soldiers. And he who hopes for success, should fight on principle, not chance. (Book 3, Foreword)”
Igitur qui desiderat pacem, praeparet bellum; qui uictoriam cupit, milites inbuat diligenter; qui secundos optat euentus, dimicet arte, non casu.
De Re Militari (also Epitoma Rei Militaris), Book III, "Dispositions for Action"
Variant: Si vis pacem para bellum. ("If you want peace, prepare for war.")

“We must be prepared to pay the price for peace, or assuredly we shall pay the price of war.”
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.

“The only way to have peace is to prepare for war.”
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

Popolo d'Italia (Feb. 1, 1921), quoted in The Menace of Fascism, John Strachey (1933) p. 65
1920s

2013, Second Inaugural Address (January 2013)