Tackling the issue of French trade unionism after the First World War entails three main difficulties: it is a contemporary issue on which each has his own pre-conceived opinions, documentation is rare and the disruption caused by the war is yet difficult to fathom. Although this history is set in an international evolution, this paper particularly seeks to bring to light the originality of the French case. It does not owe to the existence of two distinct adversaries of capitalism - socialism and trade unionism - because this duality also exists outside of France. Yet there is in France something implacable in the opposition between socialism and trade unionism. This implacability owes to the very conditions of the working proletariat, to its experience of history, its forms of organisation and its forms of action. The opinion divergences that opposed Marxists and Bakouninists - heirs of Proudhon themselves - within the First International, then Guesdists and libertarian, anti-statist and anti-political activists in the first French working class congresses, are the same as those that separated the revolutionary trade unionist movement from the socialist parliamentary movement before 1914. The class solidarity of workers founded trade unionism; the (sociologically heterogeneous) solidarity of common opinion founded the party. The party is set on taking over power by obtaining majority suffrage whereas trade unionism only relies on the direct action of the conscious minority, which is not necessarily violent and benefits the majority. The divide between revolutionaries and reformists goes through those two institutions and is not the main divide. Socialist revolutions that suppose that state control is assured and trade unionist revolutions that take root in the base, that is the autonomy of the working class organisation, must be differentiated. The French trade unionist movement seemed hesitant in the general crisis that followed the war. The war spurred as much hope as disillusion. The victories obtained through legislative means merely proved the capacity of the bourgeois state to make concessions in order to obtain the necessary production and stability. Although French trade unionism was preoccupied with the debates over the Russian Revolution, the CGT decidedly opted for the reformist tactic after the war. Bolshevism was a myth, at least according to Georges Sorel’s definition, and if the events undergone in Russia were indeed understood, it remained doubtful that they could be applied to France.