A leading theorist of the Social Democratic Party of Germany (SPD) and the Second International, Kautsky advocated orthodox Marxism, which emphasized the scientific, materialist, and determinist character of Karl Marx's work.

Kautsky's interpretation dominated European Marxism for about two decades, from the death of Friedrich Engels in 1895 to the outbreak of World War I in 1914.

He moved back to Germany in 1890 and became active in the SPD, and wrote the theory section of its Erfurt Program of 1891, a major influence on other European socialist parties.

Kautsky's stagist interpretation of Marxism emphasized that history could not be hurried, and that workers had to wait for the suitable material conditions to develop before a socialist revolution.

Under his influence, the SPD adopted a gradualist approach to achieving socialism, using bourgeois parliamentary democracy to secure improvements in the lives of workers until capitalism collapsed under its own contradictions.

His position as a prominent Marxist theorist was assured in 1888, when Engels put him to the task of editing Marx's three-volume work Theories of Surplus Value.

Following the death of Engels in 1895, Kautsky became one of the most important and influential theoreticians of Marxism, representing the mainstream of the party together with August Bebel, and outlining a Marxist theory of imperialism.

However, in June 1915, about ten months after the war had begun and when it had become obvious that this was going to be a sustained, appallingly brutal and costly struggle, he issued an appeal with Eduard Bernstein and Hugo Haase against the pro-war leaders of the SPD and denounced the German government's annexationist aims.

He saw the Bolsheviks (or Communists) as a conspiratorial organization that had gained power by a coup and initiated revolutionary changes for which there was no economic rationale in Russia.

[7] Kautsky is notable for, in addition to his anti-Bolshevik polemics, his editing and publication of Marx's Capital, Volume IV (usually published as Theories of Surplus Value).