Internationalist and defencist were the broad opposing camps in the international socialist movement during and shortly after the First World War.
The 'defencist' camp included many venerable figures of European socialism: Jules Guesde and Édouard Vaillant in France, Gustav Noske and Friedrich Ebert in Germany, Georgi Plekhanov and Ekaterina Breshkovskaia among the Russians.
Leaders of the anti-war 'Internationalist' camp included Jean Jaurès (who was murdered for his anti-war stance in 1914), Karl Liebknecht and Rosa Luxemburg, and later also Karl Kautsky and Eduard Bernstein, in Germany, Iulii Martov, Vladimir Lenin, Viktor Chernov and Mark Natanson among the Russians.
The defencist–internationalist schism did not necessarily coincide with earlier, pre-existing splits, such as that between reformists and revolutionaries, Revisionists and orthodox Marxists, Bolsheviks and Mensheviks, etc.
This was the case with Mensheviks like Fedor Dan and Irakli Tsereteli, SRs like Avram Gots and Nikolai Avksentiev and Trudoviks like Alexander Kerensky.
A small minority on the right took an out-and-out Social Patriotic stance, even supporting territorial expansion as a war aim.
Kerensky had been one of them, a Zimmerwaldist until 1917, then a Revolutionary Defencist; however, as, initially, the only socialist in the Provisional Government, he had adopted a more and more unqualified stance in support of the war, in line with his liberal colleagues.
To the left of the revolutionary defencists stood Internationalists like Chernov, who collaborated with the soviet leaders and even joined the Provisional Government, although he opposed both a continuation of the war and a coalition with the liberals.
The Left Socialist-Revolutionaries, who counted the veteran Mark Natanson and many young militants among their number, were also firmly Internationalist but broke their short-lived coalition with the Bolsheviks when the latter signed the Treaty of Brest-Litovsk.
Divisions over the war vitiated the attempts occasionally made, both before and after the October Revolution, to set up an all-socialist government, from the Bolsheviks to the Popular Socialists.
Some were motivated by unconditional pacifism, others by fascist sympathies which subsequently manifested themselves, as in the case of the ex-socialist and future premier of the Vichy régime, Pierre Laval.