The four years of war were but an interlude. In the 1920s, Langevin entered the public arena to restore international scientific relations and to build a peaceful world by using his scientific reputation. He opposed the boycott of German science that followed the Treaty of Versailles. In October 1919, he signed a petition against the blockade of the USSRUSSR. In 1921, he joined the opposition to the court-martial of the mutinous Black Sea sailors. He was curious about what was happening in the USSR, joining the Amitiés franco-russes association. Such commitments clearly marked him as “left-wing intellectual”. He joined the international movement called Clarté, founded in May 1919, comprising European intellectuals opposed to war and seeking social renewal. He was a member of the Comité international de coopération intellectuelle to promote international exchanges between academics in the sciences and humanities. In 1931, he was a member of a League of NationsLeague of Nations mission to ChinaChina to advise on the reorganization of the Chinese public education system. For him, science was a collective effort of adaptation of the human species to the outside world. It could not be nationalized.