In War: Resolution.
In Defeat: Defiance.
In Victory: Magnanimity.
In Peace: Goodwill.

Winston Churchill (1874-1965) British statesman and author

The Second World War, Vol. 1: The Gathering Storm, Epigram, “Moral of the Work,” (1948)
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The most famous use of the quote, though it can be found as far back as 21 Jan 1921 in Churchill's own hand, and Edward Marsh, his private secretary, suggested it dated back to just after WW I, as a possible war memorial.

More information here: CHURCHILL, Sir Winston Spencer (1874-1965). Autograph sentiment signed and dated, 21 January 1921, 'In War - Resolution , In Defeat - Defiance , In Victory - Magnanimity , In Peace - Goodwill', one page, 4°, in a commonplace book further inscribed and signed 'To Nell from Winston S. Churchill 21.1.21', and with one other inscription, contemporary marbled calf, front cover panelled in gilt and blind. Provenance: Roy A. Dalton; and by descent..