I do not know how the Third World War will be fought, but I can tell you what they will use in the Fourth rocks!
Albert Einstein., in an interview with Alfred Werner, Liberal Judaism 16 (April-May 1949), 12. 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: "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, 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". 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:
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!”