Hope this is substantive enough for a top level comment... There's a discussion of this at [Wikiquote] (http://en.wikiquote.org/wiki/World_War_III) that references an interview with Alfred Werner, Liberal Judaism 16 (April-May 1949) but also says references to the line or a similar paraphrased sentiment appear before then, and are also attributed to others, suggesting that Einstein may have been paraphrasing something he heard elsewhere.

One of Einstein's last political acts before his death was to sign the ["Russell-Einstein Manifesto"] (http://www.umich.edu/~pugwash/Manifesto.html) a pacifist manifesto cautioning against the development and use of nuclear weapons and warning that unless steps are taken to prevent war in general peacetime agreements to abstain from using nuclear weapons will be of limited value. That manifesto gives some insight into his thoughts on war and nuclear holocaust.