Derick Varn and Douglas Lain discuss how the social democrats split off from the socialists and how the SPD became an enemy of the really existing socialism. Of special note is the economist Rudolf Hilferding whose explanation of monopoly capitalism presaged the turn Western Marxists would take after WWII.

From Wikipedia: Rudolf Hilferding was an Austrian-born Marxist economist, leading socialist theorist, politician and chief theoretician for the Social Democratic Party of Germany (SPD) during the Weimar Republic, almost universally recognized as the SPD's foremost theoretician of his century, and a physician