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Opportunism, and the Collapse of the Second International http://www.marx2mao.com/Lenin/OCSI15a.html
The German Ideology by Marx and Engels: Saint Max https://marxistleninists.org/M&E/1845/german-ideology/ch03c.htm
The German Ideology by Marx and Engels: Saint Max https://marxistleninists.org/M&E/1845/german-ideology/ch03b.htm