Nuclear physics emerged as the dominant field in experimental and theoretical physics between 1919 and 1939, the two decades between the First and Second World Wars. Milestones were Ernest Rutherford’s discovery of artificial nuclear disintegration (1919), George Gamow’s and Ronald Gurney and Edward Condon’s simultaneous quantum-mechanical theory of alpha decay (1928), Harold Urey’s discovery of deuterium (the deuteron), James Chadwick’s discovery of the neutron, Carl Anderson’s discovery of the positron, John Cockcroft and Ernest Walton’s invention of their eponymous linear accelerator, and Ernest Lawrence’s invention of the cyclotron (1931–2), Frédéric and Irène Joliot-Curie’s discovery and confirmation of artificial radioactivity (1934), Enrico Fermi’s theory of beta decay based on Wolfgang Pauli’s neutrino hypothesis and Fermi’s discovery of the efficacy of slow neutrons in nuclear reactions (1934), Niels Bohr’s theory of the compound nucleus and Gregory Breit and Eugene Wigner’s theory of nucleus+neutron resonances (1936), and Lise Meitner and Otto Robert Frisch’s interpretation of nuclear fission, based on Gamow’s liquid-drop model of the nucleus (1938), which Frisch confirmed experimentally (1939). These achievements reflected the idiosyncratic personalities of the physicists who made them; they were shaped by the physical and intellectual environments of the countries and institutions in which they worked; and they were buffeted by the profound social and political upheavals after the Great War: the punitive postwar treaties, the runaway inflation in Germany and Austria, the Great Depression, and the greatest intellectual migration in history, which encompassed some of the most gifted experimental and theoretical nuclear physicists in the world.