I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.
Analysis
The quote is widely circulated on the internet but no primary source, such as Einstein's writings or recorded speeches, contains it. The earliest appearances are in secondary works from the 1950s and 1960s that attribute it to Einstein without documentation. Scholars and quote investigators have concluded the attribution is unfounded.
Background
Einstein did comment on the dangers of future wars, but the specific phrasing about World War IV being fought with sticks and stones is not found in his known publications or correspondence. Misattributions of similar sayings to Einstein are common, often arising from paraphrasing or apocryphal transmission.
Verdict summary
There is no credible evidence that Albert Einstein ever made this statement.
Sources consulted
— Quote Investigator: "Einstein's "World War IV" Quote" (https://quoteinvestigator.com/2014/03/03/einstein-world-war-iv/)
— Snopes: "Did Albert Einstein Say That World War IV Would Be Fought With Sticks and Stones?" (https://www.snopes.com/fact-check/einstein-world-war-iv/)
— Wikiquote: Albert Einstein – Misattributed quotations (https://en.wikiquote.org/wiki/Albert_Einstein#Misattributed_quotations)