“If we are serious about peace, then we must work for it as ardently, seriously, continuously, carefully, and bravely as we have ever prepared for war.”

1930s, Address at Chautauqua, New York (1936)

Quoted in p. 460 in Prinsep, Henry Thoby (1825). History of the political and military transactions in India during the administration of the Marquess of Hastings, 1813-1823, Vol 1. London: Kingsbury, Parbury & Allen. https://books.google.com/books?id=Tq1jAAAAMAAJ&source=gbs_navlinks_s
Quote
Context: They will not rest satisfied without establishing their own power and authority, and will unite with the hill rajas, whom we have dispossessed. We have hitherto but hunted deer; if we engage in this war, we must prepare to fight tigers.

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

"The Man Who Would Be Queen" in Melody Maker (2 May 1981) http://www.queenarchives.com/index.php?title=Freddie_Mercury_-_05-02-1981_-_Melody_Maker.

1940s, Fourth inaugural address (1945)

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.

15 April 1943
My Day (1935–1962)

Source: How to Pay for the War (1940), Ch. 1 : The Character of the Problem