V. I. Lenin

German Social-Democracy and the Right of Nations to Self-Determination {1}

German Social-Democracy was the strongest and most influential party of
the Second International. That is why, on the one hand, it bears the
greatest responsibility for its collapse, and on the other, its example and experience are most important for studying the reasons of this collapse and for analysing
the measures, ways and means for fighting the opportunism which has
strangulated that party.

The opportunism which has strangulated the Social-Democratic Party of
Germany and has transformed that party into a national-liberal labour
party, has crystallised as social-chauvinism in the 1914–15 war.