“In peace, as a wise man, he should make suitable preparation for war.”

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.")

Songs of the Soul (1971)

Quoted by Elad Benari in The Frantic Desire for Peace Only Brings War.
Source: http://www.israelnationalnews.com/News/News.aspx/140029#.VDs8U_ldUa4

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.

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

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

Source: The Inner Life

Source: Watchmen