The leader who used the slogan "Workers of the world unite!" was Karl Marx, a revolutionary socialist and co-author of "The Communist Manifesto." Written in 1848 with Friedrich Engels, this rallying cry called for the unification of the working class across national boundaries to overthrow oppressive capitalist systems. Marx's ideas laid the foundation for modern socialism and communism, advocating for the rights of workers and the idea that history is shaped by class struggles. The slogan encapsulated the drive for global solidarity among laborers, inspiring various labor movements and revolutions throughout the 19th and 20th centuries.