Nowadays, when we think about social-democracy, we have in mind a reformist movement organized by a party of officials, based on the trade-unions. This image is absolutely not conform to reality.
Social-democracy was the first organized movement of the working class, based on marxism, with socialism as goal. It was produced by and produced, dialectically, great leaders, like Karl Kautsky, Rosa Luxemburg, Bebel, Franz Mehring, Wilhelm Liebknecht, Georgi Plekhanov, Lenin, Rudolf Hilferding, Otto Bauer, Karl Renner, Wilhelm Pieck...
It gave birth to the great conception of the avant-garde party, with the leading role of ideology: Lenin is here a disciple of Kautsky. For Lenin, Kautsky was therefore in 1917 not a simple “reformist”, but a great “renegade”, because he betrayed what he stood for in the earlier period.
Without understanding social-democracy, it is so difficult to understand completely Leninism. That is why the anarchist and academic bourgeois thesis that social-democracy was a pure reformism must be rejected, like Bernstein's “revisionism” was in fact rejected from social-democracy at the end of the 19th century.
At the end of the 19th century, social-democracy had Karl Kautsky as a leader, which at that time was a true defender of Marxism, a true theoretician which tried to formulate the principle of direction for the working class.
What happened is that Engels survived Marx by 20 years. As he died in 1895, he managed to help the first Marxists to organize and to edify the social-democracy; the 10 last years Engels lived, Kautsky and him had a huge correspondence.
Karl Kautsky, born in 1854, took so the role of assuming Marxism and diffuse it in Europe. Austro-Czech, who worked early in the frame of the German social-democracy, he had so an influence in all the German speaking areas, and also in Bohemia, Moravia, Hungary, Poland, and of course Russia.
His founding of the journal Die Neue Zeit (The New Time), in 1883 (the same year Marx died), can be considered as the first main step to affirm politically Marxism as science, as it was the theoretical organ of the German social-democracy, Kautsky being the editor for 35 years.
This is not all. Not understanding social-democracy means not understanding its fierce enemy, revolutionary trade-unionism.
Whereas social-democracy understood the central value of theory and of ideological direction, revolutionary trade-unionism tried to negate this.
Revolutionary trade-unionism considered that the principle of avant-garde must be rejected, that there would be no dialectics of nature. There was only “class struggle” and to move the most masses possible, it would be necessary to struggle only on economical questions.
In the same way, for revolutionary trade-unionism, the conquest of “rights” and all democratic questions would have no “sense” and would not be in the interests of the masses.
As we know, this revolutionary trade-unionism paved the way for fascism, because of its idealism, its subjectivism, its negation of Marxism.
At the end of the 19th century, there are two ways: either orthodox Marxism, with Kautsky, or revolutionary trade-unionism with Georges Sorel, Hubert Lagardelle, Antonio Labriola, Ervin Szabó.
Basically, the more the working class was able to read Marx and Engels' works, the more it could build a strong social-democracy.
The less it was able to read Marx and Engels, in particular because of the barrier of language, the less it built a social-democracy and the more it stayed on the level of autonomous anti-political groups centered on direct economical questions.
And the only country where social-democracy managed to realize itself was Russia, with Lenin unifying social-democracy in a protracted process.