The Frisch-Peierls memorandum of March 1940 opened the way to nuclear weapons through its conclusion that the critical mass for an explosive chain reaction in 235U could be as small as 0.6 kg. This gross underestimate was based on the conjecture that the cross section for the fission of 235U with fast neutrons was 10 barns. Though this value turned out to be far too high, it is argued here that the choice made was not unreasonable at the time.