Upon the start of World War I, Slonim followed the "defensist" line of the Eser mainstream, supporting Russia's commitment to the Allies, and served in the Imperial Army. The February Revolution caught him on the Romanian Front, but he soon returned to Petrograd, where (Aucouturier writes) "his talents as a propagandist and an orator soon made him one of his party's celebrities". Slonim supported the Russian Provisional Government and its "defensist" policies against the Left Esers, with public disputations against Vladimir Karelin and Maria Spiridonova. According to Slonim, he was one of the youths left in charge of party work: the more senior Esers were either in government or consumed by work in the Soviet of Workers' and Soldiers' Deputies.