When the First World War broke out in 1914, the SRs, like most other European socialist parties, split into those who supported a war of 'national defence' ('Defencists') and those who opposed the war ('Internationalists'). Natanson sided with the 'Internationalists' and attended the international socialist peace conferences such as the Zimmerwald Conference and one at Kienthal in Switzerland, signing the conference's manifestoes on behalf of the SR Internationalists.