Souvarine experienced his first trauma with the outbreak of World War I. Mobilised as part of the French army in 1914, he quickly discovered the horrors of trench warfare and in March 1915, he lost his older brother who died fighting on the front-line.

in 1916 and began contributing to publications of the antiwar socialist minority like Le Populaire, signing articles with the pseudonym he held onto for the rest of his life.

[3] Souvarine's journalistic reputation grew rapidly during the war years as a talented, subtle writer and a skillful polemicist.

Following the Bolshevik-led revolution in November 1917, Souvarine wrote: It is to be feared that, for Lenin and his friends, the 'dictatorship of the proletariat', must be a dictatorship of Bolsheviks and their leader.

[4]In 1919, Souvarine joined the committee of the Communist International and became one of its most active members, helping to diffuse large numbers of political and propagandist literature across Europe.

In one of these leaflets, Souvarine wrote: The Socialist-Communist parties must attempt to create a proletarian democracy that will eliminate class by abolishing economic privilege, and of which the organs are soviets, i.e. peasant and worker councils—a new type of organisation governing itself.

Souvarine was arrested on 17 May 1920 in a government crackdown that accused a number of communist leaders and revolutionary activists of anarchist plots and conspiracy.

Because of a lack of substantive evidence, he was released shortly after with Fernand Loriot and Pierre Monatte, who are all acquitted in March 1921.

It was then that he composed the famous motion for the Tours Congress that would eventually split the SFIO and form the French Communist Party.

Souvarine was removed from his official roles in the French Communist Party in early 1924 and was expelled by the Comintern in July.

[6] He became close to anti-Stalinist communist figures in Paris (including Marcel Body, Christian Rakovsky and the writer Panait Istrati).

In the late 1920s, he remained active in the communist opposition, was close to Pierre Monatte and Alfred Rosmer and wrote in La Révolution Prolétarienne.