“In a modern war, fought with modern weapons and on the modern scale, neither side can limit to “the enemy” the damage that it does. These wars damage the world.”

Citizenship Papers (2003), The Failure of War

Imperialism, The Highest Stage of Capitalism (1917)

On War (1832), Book 2

Source: End Zone (1972), Ch. 16

Interview with Alfred Werner, Liberal Judaism 16 (April-May 1949), Einstein Archive 30-1104, as sourced in The New Quotable Einstein by Alice Calaprice (2005), p. 173
Differing versions of such a statement are attributed to conversations as early as 1948 (e.g. The Rotarian, 72 (6), June 1948, p. 9 http://books.google.com/books?id=0UMEAAAAMBAJ&pg=PA9: "I don't know. But I can tell you what they'll use in the fourth. They'll use rocks!"). Another variant ("I do not know with what weapons World War III will be fought, but World War IV will be fought with sticks and stones") is attributed to an unidentified letter to Harry S. Truman in "The culture of Einstein" by Alex Johnson http://www.msnbc.msn.com/id/7406337/, MSNBC, (18 April 2005). However, prior to 1948 very similar quotes were attributed in various articles to an unnamed army lieutenant, as discussed at Quote Investigator : "The Futuristic Weapons of WW3 Are Unknown, But WW4 Will Be Fought With Stones and Spears" http://quoteinvestigator.com/2010/06/16/future-weapons/#more-679. The earliest found was from “Quote and Unquote: Raising ‘Alarmist’ Cry Brings a Winchell Reply” by Walter Winchell, in the Wisconsin State Journal (23 September 1946), p. 6, Col. 3. In this article Winchell wrote: <blockquote> Joe Laitin reports that reporters at Bikini were questioning an army lieutenant about what weapons would be used in the next war. “I dunno,” he said, “but in the war after the next war, sure as Hell, they’ll be using spears!” </blockquote>
: It seems plausible, therefore, that Einstein may have been quoting or paraphrasing an expression which he had heard or read elsewhere.
1940s
Variant: I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.

Living Water

1961, UN speech
Context: Men no longer debate whether armaments are a symptom or a cause of tension. The mere existence of modern weapons — ten million times more powerful than any that the world has ever seen, and only minutes away from any target on earth — is a source of horror, and discord and distrust. Men no longer maintain that disarmament must await the settlement of all disputes — for disarmament must be a part of any permanent settlement. And men may no longer pretend that the quest for disarmament is a sign of weakness — for in a spiraling arms race, a nation's security may well be shrinking even as its arms increase.

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