“Overall, the Pox has had the effect of an installment-plan World War III. In fact, there were several small, bloody shooting wars going on around the world during the Pox. These were stupid affairs—wastes of life and treasure. They were fought, ostensibly, to defend against vicious foreign enemies. All too often, they were actually fought because inadequate leaders did not know what else to do. Such leaders knew that they could depend on fear, suspicion, hatred, need, and greed to arouse patriotic support for war.
Amid all this, somehow, the United States of America suffered a major, nonmilitary defeat. It lost no important war, yet it did not survive the Pox. Perhaps it simply lost sight of what it once intended to be, then blundered aimlessly until it exhausted itself.”

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.