In 1940, Langevin was dismissed from his professorship at the Collège de FranceCollège de Franceprofessor at and arrested by the GestapoGestapo in October. This arrest raised a wave of protests from students and colleagues. Langevin endured the torments and humiliation of jail for two months before being placed under house arrest in TroyesTroyes. He spent more than three years in this exile, deprived of his freedom of action and from his daily contacts with friends and family. He used this mandatory vacation to conduct further theoretical research, notably in the emerging field of nuclear physics. Frédéric Joliot organized his escape from Troyes in 1944 and Langevin was celebrated as a hero when he returned to Paris, especially on the occasion of his 73rd birthday in January 1945. Caught up in the enthusiasm of the tasks of reconstruction, Langevin did not feel the shock of HiroshimaHiroshima with the gravity he had given to the deadly applications of science in World War One. The chapter ends with a discussion of his optimistic response to Hiroshima against the background of the emerging French nuclear power.