“He who accepts the class struggle cannot fail to accept civil wars, which in every class society are the natural, and under certain conditions inevitable, continuation, development and intensification of the class struggle… To repudiate civil war, or to forget about it, is to fall into extreme opportunism and renounce the socialist revolution.”
As quoted in The Military Programme of the Proletarian Revolution, Collected Works, Vol. 23, pages 78-9.
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Vladimir Lenin 336
Russian politician, led the October Revolution 1870–1924Related quotes

“Lessons of the Commune”, in Zagranichnaya Gazeta, No. 2 (23 March 1908) http://www.marx.org/archive/lenin/works/1908/mar/23.htm, as translated by Bernard Isaacs, Collected Works, Vol. 13, p. 478.
1900s
Variant: The proletariat should not ignore peaceful methods of struggle — they serve its ordinary, day-to-day interests, they are necessary in periods of preparation for revolution — but it must never forget that in certain conditions the class struggle assumes the form of armed conflict and civil war; there are times when the interests of the proletariat call for ruthless extermination of its enemies in open armed clashes. This was first demonstrated by the French proletariat in the Commune and brilliantly confirmed by the Russian proletariat in the December uprising.

No Compromise – No Political Trading (1899)

"Lessons of the Commune" http://www.marx.org/archive/lenin/works/1908/mar/23.htm, Collected Works, Vol. 13.
Collected Works

“But every class struggle is a political struggle.”
Section 1, paragraph 39, lines 8-9.
The Manifesto of the Communist Party (1848)

Molotov said it in 1976 when he was in active retirement.[Molotov, Vyacheslav; Chuev, Felix; Resis, Albert, Molotov remembers: inside Kremlin politics : conversations with Felix Chuev, I.R. Dee, 1993, 1566630274, 20]

Collected Works, Vol. 26, pp. 379–383.
Collected Works