This issue of Socialist History brings together a collection of articles that highlight the importance of continued historical (re-)investigation and debate, and the evolution of historical thought on the political left. Francis King looks at the Zimmerwald Manifesto and a seemingly intractable contradiction that it created for Russian socialists in 1917. Pawel Szelegieniec considers the implementation of Stalinism and the conflict that it created between an economically centralising state and an increasingly resistant proletariat on whose behalf it governed. Our third article is a posthumous essay by the late John Saville. A fiercely anti-colonial piece by a significant figure of the British left, it is simultaneously a piece of historical writing and an historical source. Finally, David Renton considers the evolution of regionalism as a theme within the writing of another significant socialist historian: Sidney Pollard.