This is the first biography of Sir James Chadwick (1891-1974), Nobel Laureate and discoverer of the neutron. His central role in the unfolding drama of nuclear physics is reflected in his publications and his correspondence with leading figures like Bohr and Rutherford. While Chadwick changed the course of science, his life was shaped by great events. He was studying with Geiger in Berlin when war broke out in 1914, and spent the next four years in an internment camp: despite immense hardships, he built a scientific laboratory and continued to experiment. At the start of World War Two he narrowly avoided being trapped in Europe again. His pre-eminence as a physicist meant that he was soon consulted about the feasibility of an atomic bomb: he co-ordinated the initial research at several British universities. In 1943 he made his first trip to the USA, where he became chief British scientist on the Manhattan Project. In this capacity, he formed an unlikely friendship with General Groves, who came to trust him implicitly. Together they were at the centre of several international controversies that threatened Anglo-American relations. His career exemplifies the loss of innocence in twentieth-century science, a process inadvertently hastened by the discovery of the neutron.