On the left the theory of peace was influenced more by Karl Marx than Adam Smith. Where liberals emphasized free trade and economic interdependence, socialists focused on questions of equality and economic justice. They criticized rather than praised capitalism, exposing the economic motives of imperialism, and the bellicose influence of munitions makers. They agreed that interdependence promoted peace, but they had a very different model of socialist solidarity in mind. Gradually the two movements began to converge, as peace advocates increasingly embraced social justice concerns. In many respects a natural affinity existed between socialism and pacifism. Socialism was quintessentially an international movement, as reflected in the title of its famous anthem. Marx emphasized the transnational character of class society and the need for a global solution to the problems of injustice and war. Peace and justice are inextricably linked, socialists insisted. An end to economic exploitation and imperialism is necessary for genuine peace. Many pacifists agreed with this analysis and incorporated the struggle against inequality into the peace agenda.

There were also sharp differences between socialism and pacifism. Marx opposed capitalist war but favored the class war. Support for armed struggle was a defining feature of socialism in its early years, especially in the communist Third International, and it remains so today among Trotskyists, Maoists, and other communist groups.