Proletarian internationalists now argued that the alliances of the First World War had proved that socialism and nationalism were incompatible in the imperialist era, that the concept of national self-determination had become outdated, and in particular, that nationalism would prove to be an obstacle to proletarian unity. Anarcho-syndicalism was a further working class political current that characterised the war as imperialist on all sides, finding organisational expression in the Industrial Workers of the World.