V.I. Lenin
Vol.21 Collected Works, p. 311
"Socialism and War"
1915

"In the past, before the war, opportunism was often 
looked upon as a legitimate, though 'deviationist'
and 'extremist,' component of the Social-
Democratic Party.  The war has shown the 
impossibility of this in the future.  Opportunism 
has 'matured,' and is now playing to the full its 
role as emissary of the bourgeoisie in the 
working-class movement.  Unity with the 
opportunists has become sheer hypocrisy, 
exemplified by the German Social-Democratic Party. 
On every important occasion (e.g., the August 4 
vote), the opportunists present an ultimatum, to 
which they give effect through their numerous 
links with the bourgeoisie, their majority on the 
executives of the trade unions, etc.  Today unity 
with the opportunists actually means subordinating 
the working class to their 'own' national 
bourgeoisie, and an alliance with the latter for 
the purpose of oppressing other nations and of 
fighting for dominant-nation privileges; it means 
splitting the revolutionary proletariat of all 
countries."