“Ladies and gentlemen: War may sometimes be a necessary evil. But no matter how necessary, it is always an evil, never a good. We will not learn how to live together in peace by killing each other's children.”

Fussell here slightly paraphrases Hemingway's statement from his Foreword to Treasury for the Free World (1946): Never think that war, no matter how necessary nor how justified, is not a crime. Ask the infantry and ask the dead.
Humanities interview (1996)

As quoted in Margaret Mead : Some Personal Views (1979) edited by Rhoda Métraux
As quoted in American Quotations (1992) by Gorton Carruth and Eugene H. Ehrlich
1970s
Variant: At times it may be necessary temporarily to accept a lesser evil, but one must never label a necessary evil as good.

Introduction to Treasury of the Free World (1946)
Source: Ernest Hemingway: A Literary Reference
Context: An aggressive war is the great crime against everything good in the world. A defensive war, which must necessarily turn to aggressive at the earliest moment, is the necessary great counter-crime. But never think that war, no matter how necessary, nor how justified, is not a crime. Ask the infantry and ask the dead.

Source: The Infernals

2015, Commemoration of the 150th Anniversary of the 13th Amendment (December 2015)

Source: Losing My Virginity: How I've Survived, Had Fun, and Made a Fortune Doing Business My Way

Source: Aphorisms and Reflections (1901), pp. 129-130

Churchill By Himself: The Definitive Collections of Quotations, ed. Richard Langworth, 2008, p. 424, (1907, 12 February)
Early career years (1898–1929)