Friedrich Engels greatly influenced socialist theory, primarily through his collaboration with Karl Marx. Engels co-authored "The Communist Manifesto," where he articulated the struggles of the working class against capitalist systems. His analysis of capitalism in "The Condition of the Working Class in England" shed light on the harsh realities of industrial life, reinforcing the need for socialist reform. Engels also funded Marx's work, ensuring the sustainability of their revolutionary ideas. His contributions shaped foundational aspects of communist ideology and provided a framework for assessing class relations and economic systems throughout history.