The Frisch–Peierls memorandum was the first technical exposition of a practical nuclear weapon. It was written by expatriate German physicists Otto Frisch and Rudolf Peierls in March 1940 while they were both working for Mark Oliphant at the University of Birmingham in Britain during World War II.
The memorandum contained the first calculations about the size of the critical mass of fissile material needed for an atomic bomb. It revealed for the first time that the amount required might be small enough to incorporate into a bomb that could be delivered by air. It also anticipated the strategic and moral implications of nuclear weapons.
It helped send both Britain and America down a path which led to the MAUD Committee, the Tube Alloys project, the Manhattan Project, and ultimately the atomic bombings of Hiroshima and Nagasaki.
https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memorandum