This book analyzes a little-known but highly significant document that played a key role in early nuclear history: the Frisch-Peierls memorandum of March 1940. Prepared by Otto Frisch and Rudolf Peierls, refugee European physicists then at the University of Birmingham, this 10-page document described the physics behind the possibility of creating nuclear weapons utilizing a chain reaction with uranium-235, as well as the associated military, strategic, and ethical implications of such weapons. This remarkable manuscript made its way to the UK government's Committee on the Scientific Survey of Air Warfare, initiating the wartime British nuclear program. In 1943, the British effort merged with the US Manhattan Project; a number of native and naturalized British scientists including Frisch and Peierls, participated in the work that culminated with the bombings of Hiroshima and Nagasaki. This book examines the background to the memorandum, gives biographical sketches of Frisch and Peierls, describes how the memorandum came to be prepared, offers a detailed analysis of its physics content, outlines contemporary parallel events in the American nuclear program, and surveys the influence of the memorandum on the British and US wartime nuclear projects.