In this work, Lenin defends the international and revolutionary essence of Marxism at a time when the majority of the leaders of the Second International were contorting Marx’s teachings into a doctrine of reformism and lining up behind their ruling classes in the First World War.
From the preface:

This article on Karl Marx, which now appears in a separate printing, was written in 1913 (as far as I can remember) for the Granat Encyclopaedia. A fairly detailed bibliography of literature on Marx, mostly foreign, was appended to the article. This has been omitted in the present edition. The editors of the Encyclopaedia, for their part, have, for censorship reasons, deleted the end of the article on Marx, namely, the section dealing with his revolutionary tactics. Unfortunately, I am unable to reproduce that end, because the draft has remained among my papers somewhere in Krakow or in Switzerland. I only remember that in the concluding part of the article I quoted, among other things, the passage from Marx’s letter to Engels of April 16, 1856, in which he wrote: “The whole thing in Germany will depend on the possibility of backing the proletarian revolution by some second edition of the Peasant War. Then the affair will be splendid.” That is what our Mensheviks, who have now sunk to utter betrayal of socialism and to desertion to the bourgeoisie, have failed to understand since 1905.

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