 At the outbreak of World War I, the SRs (like other revolutionary parties) split into 'Defencist' and 'Internationalist' antiwar groups; Rudnev, like his colleagues AA Argunov and ND Avksentiev, took the former position, in opposition to the party's leaders, Victor Chernov and Mark Natanson, and worked as a doctor on a hospital ship.During the February Revolution he was a leader of the Moscow branch of the party and edited its newspaper "Trud", and in July he was chosen mayor of Moscow; he supported the policies of Alexander Kerensky.He opposed the October Revolution, and after the dispersal of the Constituent Assembly (of which he was a member) he fled south, first to Kiev, then the Caucasus, and finally Odessa.