“Most Americans mistakenly believe that peace is the normal condition of mankind, while war is some weird aberration. Actually, it’s the other way around. Most of human history has to do with war, and preparations for the next one. But we Americans do not prepare for the next war, are invariably surprised when it erupts, and, since we did not take prudent steps when it would have been relatively simple to prevail, usually end up fighting on our enemies’ more difficult and costly terms.”

1960s, A Time for Choosing (1964)
Context: As for the peace that we would preserve, I wonder who among us would like to approach the wife or mother whose husband or son has died in South Vietnam and ask them if they think this is a peace that should be maintained indefinitely. Do they mean peace, or do they mean we just want to be left in peace? There can be no real peace while one American is dying some place in the world for the rest of us. We're at war with the most dangerous enemy that has ever faced mankind in his long climb from the swamp to the stars, and it's been said if we lose that war, and in so doing lose this way of freedom of ours, history will record with the greatest astonishment that those who had the most to lose did the least to prevent its happening. Well I think it's time we ask ourselves if we still know the freedoms that were intended for us by the Founding Fathers.

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

The Manchester Guardian (28 May 1934), quoted in Hugh Dalton, The Fateful Years. Memoirs 1931-1945 (London: Frederick Muller Ltd, 1957), p. 150.

Special Message to the Congress on the Threat to the Freedom of Europe (1948)

1963, American University speech

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

Peace and the Public Mind (1935)

Source: Watchmen

Source: Unseen Academicals