Karl Marx's collaborator and co-author of "The Communist Manifesto" was Friedrich Engels. Together, they established the foundation of Marxist theory, critiquing the capitalist system and advocating for a classless society. Engels provided substantial support to Marx, both intellectually and financially, and their collaboration significantly shaped modern political thought. Their manifesto, published in 1848, presents a call to arms for the working class, urging them to unite against oppression and fight for their rights. The work remains influential in discussions of socialism and communism around the world.