“For war there is always enough. It's peace that's expensive.”

"Poetry in War and Peace," Partisan Review (Winter 1945) [p. 129]
Kipling, Auden & Co: Essays and Reviews 1935-1964 (1980)

1960s, The Quest for Peace and Justice (1964)
Context: We will not build a peaceful world by following a negative path. It is not enough to say "We must not wage war." It is necessary to love peace and sacrifice for it. We must concentrate not merely on the negative expulsion of war, but on the positive affirmation of peace. There is a fascinating little story that is preserved for us in Greek literature about Ulysses and the Sirens. The Sirens had the ability to sing so sweetly that sailors could not resist steering toward their island. Many ships were lured upon the rocks, and men forgot home, duty, and honor as they flung themselves into the sea to be embraced by arms that drew them down to death. Ulysses, determined not to be lured by the Sirens, first decided to tie himself tightly to the mast of his boat, and his crew stuffed their ears with wax. But finally he and his crew learned a better way to save themselves: they took on board the beautiful singer Orpheus whose melodies were sweeter than the music of the Sirens. When Orpheus sang, who bothered to listen to the Sirens? So we must fix our vision not merely on the negative expulsion of war, but upon the positive affirmation of peace. We must see that peace represents a sweeter music, a cosmic melody that is far superior to the discords of war.

2009, Nobel Prize acceptance speech (December 2009)
Context: I understand why war is not popular, but I also know this: The belief that peace is desirable is rarely enough to achieve it. Peace requires responsibility. Peace entails sacrifice. That's why NATO continues to be indispensable. That's why we must strengthen U. N. and regional peacekeeping, and not leave the task to a few countries.

It is necessary to love peace and sacrifice for it. We must concentrate not merely on the negative expulsion of war, but on the positive affirmation of peace. There is a fascinating little story that is preserved for us in Greek literature about Ulysses and the Sirens. The Sirens had the ability to sing so sweetly that sailors could not resist steering toward their island. Many ships were lured upon the rocks, and men forgot home, duty, and honor as they flung themselves into the sea to be embraced by arms that drew them down to death. Ulysses, determined not to be lured by the Sirens, first decided to tie himself tightly to the mast of his boat, and his crew stuffed their ears with wax. But finally he and his crew learned a better way to save themselves: they took on board the beautiful singer Orpheus whose melodies were sweeter than the music of the Sirens. When Orpheus sang, who bothered to listen to the Sirens? So we must fix our vision not merely on the negative expulsion of war, but upon the positive affirmation of peace. We must see that peace represents a sweeter music, a cosmic melody that is far superior to the discords of war.
1960s, The Quest for Peace and Justice (1964)

As quoted in "Is World Peace on the Horizon?", in The Watchtower (15 April 1991)

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

"When War Drums Roll" (17 September 2001)
2000s
Context: This is going to be a very expensive war, and Victory is not guaranteed — for anyone, and certainly not for anyone as baffled as George W. Bush. All he knows is that his father started the war a long time ago, and that he, the goofy child-President, has been chosen by Fate and the global Oil industry to finish it. Now.