The first duty of revolutionaries in the face of capitalist war is to defend the interests of the whole working class, as expressed in the historic slogan of the workers' movement: "Workers of the world unite!". The defence of internationalism for revolutionaries has never been an abstract principle; it is an intensely practical struggle, involving a fight for clarity inside the political organisations of the working class, and more widely through intervention in its defensive struggles, often in difficult conditions of state repression and patriotic frenzy. This series, written by a sympathiser of the ICC, traces the development of a number of political currents within Britain through the period from the First World War, the Third International, the Second World War, and finally the enlisting of the Trotskyists in this war.