“Happy the state which in times of peace is yet prepared for war.”

Section 2, member 6, Perturbations of the mind rectified. From himself, by resisting to the utmost, confessing his grief to a friend, etc.
The Anatomy of Melancholy (1621), Part II

"A Statement against the War in Vietnam".
The Long-Legged House (1969)

Source: Unseen Academicals

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

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

Book II, satire ii, line 111
Satires (c. 35 BC and 30 BC)

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.

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.

Final lines
Civil Disobedience (1849)
Context: Is a democracy, such as we know it, the last improvement possible in government? Is it not possible to take a step further towards recognizing and organizing the rights of man? There will never be a really free and enlightened State until the State comes to recognize the individual as a higher and independent power, from which all its own power and authority are derived, and treats him accordingly. I please myself with imagining a State at least which can afford to be just to all men, and to treat the individual with respect as a neighbor; which even would not think it inconsistent with its own repose if a few were to live aloof from it, not meddling with it, nor embraced by it, who fulfilled all the duties of neighbors and fellow-men. A State which bore this kind of fruit, and suffered it to drop off as fast as it ripened, would prepare the way for a still more perfect and glorious State, which also I have imagined, but not yet anywhere seen.