“Either Man will abolish war, or war will abolish Man.”

Nobel Lecture for The Nobel Peace Prize 1962 (11 December 1963).
1940s-1960s

James Hinton, Philosophy and Religion: Selections from the Manuscripts of the Late James Hinton, ed. Caroline Haddon, (2nd ed., London: 1884), [//books.google.com/books?id=DpxRAAAAMAAJ&pg=PA267 p. 267].
Widely misattributed on the internet to Dewey, who actually attributes it to Hinton in Human Nature and Conduct: An Introduction to Social Psychology (New York: 1922), [//books.google.com/books?id=Ws0RAAAAYAAJ&pg=PA115 p. 115].
Misattributed

"Father Severyan", in November 1916: The Red Wheel: Knot II (1984; translation 1999).
Context: At no time has the world been without war. Not in seven or ten or twenty thousand years. Neither the wisest of leaders, nor the noblest of kings, nor yet the Church — none of them has been able to stop it. And don't succumb to the facile belief that wars will be stopped by hotheaded socialists. Or that rational and just wars can be sorted out from the rest. There will always be thousands of thousands to whom even such a war will be senseless and unjustified. Quite simply, no state can live without war, that is one of the state's essential functions. … War is the price we pay for living in a state. Before you can abolish war you will have to abolish all states. But that is unthinkable until the propensity to violence and evil is rooted out of human beings. The state was created to protect us from evil. In ordinary life thousands of bad impulses, from a thousand foci of evil, move chaotically, randomly, against the vulnerable. The state is called upon to check these impulses — but it generates others of its own, still more powerful, and this time one-directional. At times it throws them all in a single direction — and that is war.

In response to journalist for his views on the future of mankind at his 70th birthday (16 April 1959)

A Great Experiment (1941), p. 189
Context: The truth is, I was never a very good Party man. Probably but for the War of 1914, I should have gone on fairly comfortably as a Conservative official. But those four years burnt into me the insufferable conditions of international relations which made war the acknowledged method — indeed, the only fully authorized method — of settling international disputes. Thenceforth, the effort to abolish war seemed to me, and still seems to me, the only political object worth while.

An American Peace Policy (1925)

1940s, To Every Briton (1940)
Context: This war has descended upon mankind as a curse and a warning. It is a curse inasmuch as it is brutalizing man on a scale hitherto unknown. All distinctions between combatants and noncombatants have been abolished. No one and nothing is to be spared. Lying has been reduced to an art. Britain was to defend small nationalities. One by one they have vanished, at least for the time being. It is also a warning. It is a warning that, if nobody reads the writing on the wall, man will be reduced to the state of the beast, whom he is shaming by his manners. I read the writing when the hostilities broke out. But I had not the courage to say the word. God has given me the courage to say it before it is too late.

Politics, Volume I, p. 74.

Source: Animal Farm