The theories most often appealed to by labor unions to justify their actions are the theories of Karl Marx. Marx developed a critical analysis of capitalism, arguing that it exploited workers and created a class struggle between the capitalists who owned the means of production and the working-class laborers who did not. Marx believed that the working class should overthrow the capitalist class and establish a socialist society in which workers collectively own and control the means of production.
Labor unions often use Marx's theories to justify their actions, such as striking for better wages, benefits, and working conditions. They argue that the capitalist system exploits workers by paying them less than the value of their labor and that collective action is necessary to achieve greater economic and social justice.
It's worth noting that not all labor unions subscribe to Marx's theories, and there are many different perspectives on the role and purpose of unions in society. However, Marx's ideas about labor and capitalism have had a significant influence on the history and development of labor unions around the world.