of internationalism are known as internationalists and generally believe that humans should unite across national, political, cultural, racial, or class boundaries to advance their common interests. Internationalism is characterized by opposition to nationalism and isolationism, support for international struggles, and a cosmopolitan outlook that promotes and respects other cultures. No Borders No Nations is a popular internationalist slogan in the anarchist movement.

Socialist thinkers such as Karl Marx and Friedrich Engels argue that economic class, rather than nationality, race, or culture, is the main force that divides people in society and that nationalist ideology is a propaganda tool of a society's dominant economic class. From this perspective, it is in the ruling class' interest to promote nationalism in order to hide the inherent class conflicts at play within a given society, such as the exploitation of workers by capitalists for profit. Therefore, socialists see nationalism as a form of ideological control arising from a society's given mode of economic production. Since the 19th century, socialist political organizations and radical trade unions such as the Industrial Workers of the World (IWW) have promoted internationalist ideologies and sought to organize workers across national boundaries to achieve improvements in the conditions of labor and advance various forms of industrial democracy. The First, Second, Third, and Fourth Internationals were socialist political groupings that sought to advance worker's revolution across the globe and achieve international socialism and world revolution.

Socialist internationalism is anti-imperialist, and therefore supports the liberation of peoples from all forms of colonialism and foreign domination and the right of nations to self-determination. Therefore, socialists have often aligned themselves politically with anti-colonial independence movements, and actively opposed the exploitation of one country by another. Proponents of proletarian internationalism often argued that the objectives of a given revolution should be global rather than local in scope—for example, triggering or perpetuating revolutions elsewhere. Proletarian internationalism is closely linked to goals of world revolution, to be achieved through successive or simultaneous communist revolutions in all nations. According to Marxist theory, successful proletarian internationalism should lead to a world revolution and eventually stateless communism. The notion was strongly embraced by the first communist party, the Communist League, as exercised through its slogan "Proletarians of all countries, unite!", later popularized as "Workers of the world, unite!" in English literature.